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MonSeptember 17, 2007

Science & Nature10 Easy Arithmetic Tricks418 Comments

386Like

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Math can be terrifying for many people. This list will hopefully improve your general knowledge of mathematicaltricks and your speed when you need to do math in your head.

10 Easy Arithmetic Tricks http://listverse.com/2007/09/17/10-easy-arithmetic-tricks/

1 of 24 3/19/2011 1:33 PM

1. The 11 Times Trick

We all know the trick when multiplying by ten – add 0 to the end of the number, but did you know there is an

equally easy trick for multiplying a two digit number by 11? This is it:Take the original number and imagine a space between the two digits (in this example we will use 52:

5_2Now add the two numbers together and put them in the middle:

5_(5+2)_2

That is it – you have the answer: 572.If the numbers in the middle add up to a 2 digit number, just insert the second number and add 1 to the first:

9_(9+9)_9(9+1)_8_9

10_8_9

1089 – It works every time.2. Quick Square

If you need to square a 2 digit number ending in 5, you can do so very easily with this trick. Mulitply the first digitby itself + 1, and put 25 on the end. That is all!

252 = (2x(2+1)) & 25

2 x 3 = 6625

3. Multiply by 5Most people memorize the 5 times tables very easily, but when you get in to larger numbers it gets more complex

– or does it? This trick is super easy.

Take any number, then divide it by 2 (in other words, halve the number). If the result is whole, add a 0 at the end.If it is not, ignore the remainder and add a 5 at the end. It works everytime:

2682 x 5 = (2682 / 2) & 5 or 02682 / 2 = 1341 (whole number so add 0)

13410

Let’s try another:5887 x 5

2943.5 (fractional number (ignore remainder, add 5)29435


2 of 24 3/19/2011 1:33 PM

4. Multiply by 9

This one is simple – to multiple any number between 1 and 9 by 9 hold both hands in front of your face – drop the

finger that corresponds to the number you are multiplying (for example 9×3 – drop your third finger) – count thefingers before the dropped finger (in the case of 9×3 it is 2) then count the numbers after (in this case 7) – the

answer is 27.5. Multiply by 4

This is a very simple trick which may appear obvious to some, but to others it is not. The trick is to simply

multiply by two, then multiply by two again:58 x 4 = (58 x 2) + (58 x 2) = (116) + (116) = 232

6. Calculate a Tip

If you need to leave a 15% tip, here is the easy way to do it. Work out 10% (divide the number by 10) – then addthat number to half its value and you have your answer:

15% of $25 = (10% of 25) + ((10% of 25) / 2)

$2.50 + $1.25 = $3.757. Tough Multiplication

If you have a large number to multiply and one of the numbers is even, you can easily subdivide to get to theanswer:

32 x 125, is the same as:

16 x 250 is the same as:8 x 500 is the same as:

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3 of 24 3/19/2011 1:33 PM

4 x 1000 = 4,000

8. Dividing by 5Dividing a large number by five is actually very simple. All you do is multiply by 2 and move the decimal point:

195 / 5

Step1: 195 * 2 = 390Step2: Move the decimal: 39.0 or just 39

2978 / 5step 1: 2978 * 2 = 5956

Step2: 595.6

9. Subtracting from 1,000To subtract a large number from 1,000 you can use this basic rule: subtract all but the last number from 9, then

subtract the last number from 10:1000

-648

step1: subtract 6 from 9 = 3step2: subtract 4 from 9 = 5

step3: subtract 8 from 10 = 2answer: 352

10. Assorted Multiplication Rules

Multiply by 5: Multiply by 10 and divide by 2.Multiply by 6: Sometimes multiplying by 3 and then 2 is easy.

Multiply by 9: Multiply by 10 and subtract the original number.Multiply by 12: Multiply by 10 and add twice the original number.

Multiply by 13: Multiply by 3 and add 10 times original number.

Multiply by 14: Multiply by 7 and then multiply by 2Multiply by 15: Multiply by 10 and add 5 times the original number, as above.

Multiply by 16: You can double four times, if you want to. Or you can multiply by 8 and then by 2.Multiply by 17: Multiply by 7 and add 10 times original number.

Multiply by 18: Multiply by 20 and subtract twice the original number (which is obvious from the first step).

Multiply by 19: Multiply by 20 and subtract the original number.Multiply by 24: Multiply by 8 and then multiply by 3.

Multiply by 27: Multiply by 30 and subtract 3 times the original number (which is obvious from the first step).Multiply by 45: Multiply by 50 and subtract 5 times the original number (which is obvious from the first step).

Multiply by 90: Multiply by 9 (as above) and put a zero on the right.

Multiply by 98: Multiply by 100 and subtract twice the original number.Multiply by 99: Multiply by 100 and subtract the original number.

Bonus: PercentagesYanni in comment 23 gave an excellent tip for working out percentages, so I have taken the liberty of duplicating

it here:


4 of 24 3/19/2011 1:33 PM

Share

Find 7 % of 300. Sound Difficult?

Percents: First of all you need to understand the word “Percent.” The first part is PER , as in 10 tricks perlistverse page. PER = FOR EACH. The second part of the word is CENT, as in 100. Like Century = 100 years.

100 CENTS in 1 dollar… etc. Ok… so PERCENT = For Each 100.So, it follows that 7 PERCENT of 100, is 7. (7 for each hundred, of only 1 hundred).

8 % of 100 = 8. 35.73% of 100 = 35.73

But how is that useful??Back to the 7% of 300 question. 7% of the first hundred is 7. 7% of 2nd hundred is also 7, and yep, 7% of the 3rd

hundred is also 7. So 7+7+7 = 21.If 8 % of 100 is 8, it follows that 8% of 50 is half of 8 , or 4.

Break down every number that’s asked into questions of 100, if the number is less then 100, then move the

decimal point accordingly.EXAMPLES:

8%200 = ? 8 + 8 = 16.8%250 = ? 8 + 8 + 4 = 20.

8%25 = 2.0 (Moving the decimal back).

15%300 = 15+15+15 =45.15%350 = 15+15+15+7.5 = 52.5

Also it’s usefull to know that you can always flip percents, like 3% of 100 is the same as 100% of 3.35% of 8 is the same as 8% of 35.

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Reea / 17 Sep, 2007 at 04:07 amI always sucked at math, this is useful, another interesting list, love your site!

Reply

jfrater / 17 Sep, 2007 at 04:27 amReea: same here – this list stems from my trying to find something to help me fix my mess of a mathematical

brain.

Reply

tony3s / 17 Sep, 2007 at 05:02 am

I didn’t know about trick number 10.And it seems there is a good reason for that, because it doesn’t work !

e.g.: 58 / 9 = 6r4

but according to the trick: 5r4e.g.: 587 / 9 = 65r2

but according to the trick: 54r2So this trick doesn’t work all the time.

The only thing that’s true is that summing all the numbers gives the remainder (I knew that).

But good post to remind people math doesn’t have to be hard. Reply

Graeme / 28 May, 2010 at 06:40 pm

for starters you need to learn to read, trick number 10 = 10. Assorted Multiplication Rulesas far as i can remember 58 / 9 = division?

Reply

jfrater / 17 Sep, 2007 at 05:39 am

tony3s: You are right – I have replaced 10 with assorted multiplication rules because the previous number 10 does

only work in certain circumstances.Reply

dalandzadgad / 17 Sep, 2007 at 06:12 am

awesome list. i inadvertently learned some of the tricks listed here. but i used to memorize math…Reply

blazak / 17 Sep, 2007 at 10:00 amThe trick for multiplying by 9 is good, but I always thought you’d look like an idiot in math class staring at your

fingers, so my trick was simply…the answer will add up to 9, with the first digit being one less than your original

multiplier…ex – 7*9 = 63 (6 is one less than 7, and 6+3=9)

Reply


8 of 24 3/19/2011 1:33 PM


nice addition blazak.Reply

Dan / 17 Sep, 2007 at 12:04 pm

I prefer calculators… but I do know a few of these tricks and they are helpful when I don’t have one.Reply

The China Tattler / 17 Sep, 2007 at 12:39 pmMy brain freezes when numbers and words are combined.

I know most of these things, I can do them without thinking. But, when I read how to do them, I just zone out.

It just seems too confusing, like trying to translate English into Chinese characters using pinyin.Math has its own language.

Reply

Shad / 17 Sep, 2007 at 12:40 pm#3… 2682 / 2 = 1341 (even number so add 0) …

Isn’t 1341 an odd number? the math still works, but I’m all sorts of confused now. And I’ve always sucked atmath

Reply

Michael F. Cox / 11 Aug, 2010 at 09:43 pmIt's WHOLE NUMBERS vs. FRACTIONAL NUMBERS, not Odd vs. Even.

(Maybe it has been updated since your comment.)Reply

Tom Steele / 17 Sep, 2007 at 12:43 pm

Neat stuff, but #3 appears to be incorrect.It says if you have an even number, you add a zero to the end and if you have an odd number you add a 5 to the

end.I believe they mean to say, “if you have a WHOLE number add a zero to the end, and if you have a

fraction/decimal then add a 5 to the end.”

1341 is an ODD number, not an even number. But it IS a WHOLE number.Reply

jfrater / 17 Sep, 2007 at 01:01 pm

shad and Tom: thanks – corrected.Reply

Strath / 17 Sep, 2007 at 01:43 pm#3 is just the reverse of #8. Divide by 2 and shift the decimal place. No need to remember even/odd

whole/fractional rule.

2682 * 5


9 of 24 3/19/2011 1:33 PM

Step1: 2682 / 2 = 1341

Step2: Move the decimal: 134105887 * 5

Step1: 5887 / 2 = 2943.5Step2: Move the decimal: 29435

Reply

Strath / 17 Sep, 2007 at 01:51 pmOh, and #1, simpler method to multiply by 11 and it works for any number of digits.

Multiply by 10 and add the original number.Reply


Strath: thanks for the additions Reply

anteater_sa / 17 Sep, 2007 at 04:12 pm

Some cool tricks here, heres another, to find if a number is a multiple of 3, add all the numbers together and ifthey are evenly divisable by 3 then it is. e.g: 9576 –> 9 + 5 + 7 + 6 = 27 therefore 9576 is a multiple of 3

Reply

ow3n / 17 Sep, 2007 at 04:30 pmOne trick I use that I didn’t see listed is multiplying large numbers.

Like:12 x 734

can be broken into ((10×700) + (2×700) + (10×34) + (2×34)) = 7068

or8 x 6846

can be broken into ((8×6000) + (8×800) + (8×40) + (8×6)) = 54768basically you break the number into tens, hundreds, thousands, etc and mulitply each group. with some larger

numbers it can get hard to remember each “sub-number” but if you’re trying to estimate, its a good way to get a

rough idea.great post otherwise, i learned a lot of tips i wouldn’t have thought of otherwise..

~ow3nReply

ben / 17 Sep, 2007 at 05:26 pm

I wish I would have had this a few years ago! Now im in high school calculus, its a little harder than these rules

apply still great!

Reply

Miss Cellania / 17 Sep, 2007 at 05:54 pmTo multiply something by five, I always multiplied by ten, then halved the answer.

Reply


10 of 24 3/19/2011 1:33 PM

Fe / 17 Sep, 2007 at 06:47 pm

Gotta agree with China Tattler, trying to read how to perform math tricks is like trying to sing a fragrance, it justdoesn’t work for me. I worked in a fabric store for four years and pretty quick picked up all sorts of tricks to

calculate percentages, discounts, fractions and decimals all in my head, but explaining them confuses just abouteveryone.

Reply

Ben / 17 Sep, 2007 at 07:49 pmHere is a trick…

I turn on the calculator and start typing!

JKThese are great tricks essp for the kids still in high school and middle school. The joys of college math is the same

but you dont use numbers just english and greek letters.Reply

penny / 17 Sep, 2007 at 09:36 pm

A better system:The Tractenberg System Of Speed Math.

Also check out:Dead Reckoning

For advanced tricks–such as superspeed Langrange

Division.Reply

Yanni / 17 Sep, 2007 at 11:24 pmWe teach many of these “tricks” in our Mathnasium Learning Center (www.mathnasium.com). Sorry for the plug,

but here are some interesting “tricks.”

Find 7 % of 300. Sound Difficult?Percents: First of all you need to understand the word “Percent.” The first part is PER , as in 10 tricks per

listverse page. PER = FOR EACH. The second part of the word is CENT, as in 100. Like Century = 100 years.100 CENTS in 1 dollar… etc. Ok… so PERCENT = For Each 100.

So, it follows that 7 PERCENT of 100, is 7. (7 for each hundred, of only 1 hundred).

8 % of 100 = 8. 35.73% of 100 = 35.73But how is that useful??

Back to the 7% of 300 question. 7% of the first hundred is 7. 7% of 2nd hundred is also 7, and yep, 7% of the 3rdhundred is also 7. So 7+7+7 = 21.

If 8 % of 100 is 8, it follows that 8% of 50 is half of 8 , or 4.

Break down every number that’s asked into questions of 100, if the number is less then 100, then move thedecimal point accordingly.

EXAMPLES:8%200 = ? 8 + 8 = 16.

8%250 = ? 8 + 8 + 4 = 20.

8%25 = 2.0 (Moving the decimal back).15%300 = 15+15+15 =45.

15%350 = 15+15+15+7.5 = 52.5Also it’s usefull to know that you can always flip percents, like 3% of 100 is the same as 100 %of 3.

35% of 8 is the same as 8% of 35.


11 of 24 3/19/2011 1:33 PM

Reply

Michael F. Cox / 11 Aug, 2010 at 10:04 pmYou say "35% of 8 is the same as 8% of 35."

That's because Multiplication is Associative, meaning that the ORDERin which you group the items to be multiplied is wholly irrelevant.

(0.35 times is the same as (35 times 0.08)

So saying that 35% of 8 is the same as 8% of 35 is simply"associating" the .01 (the decimal equivalent of 1%) first with

the 35 and then alternatively, with the 8.

Premise:( 35 percent ) * 8 = 35 * ( 8 percent )

Proof:( 35 * 0.01 ) * 8 = 35 * ( 0.01 * 8 )

Moot point: The above equation is equal to [ ( 8 * 35) * 0.01 ]

Reply


Yanni: That is the most useful mathematics tip I have ever heard. Thanks for adding it – brilliant!Reply

qureyoon / 18 Sep, 2007 at 12:45 am

this is amazing ! i only knew few of these tricksthanks for sharing ^^

Reply

Andre du Plessis / 18 Sep, 2007 at 01:26 am1, 2 and 3 I knew about (you can excercise them with the following feed – http://www.mental-workout.com/)

about 1 : I also worked out you can do the same with 3 numbers eg.172 * 11

1(1+7)(7+2)2 = 1892

I am pretty sure you will be able expand on that even further.5 and 6 did seem obvious to me.

9 I would rather go to the nearest 100 eg. 1000 – 700 = 300 and then do an easy 100 subtraction 100 – 48 = 52then add them up 300 +52 = 352 (may seem longer but actually goes pretty fast in my mind)

Thanks for the others

Reply

Dhammika / 23 Jun, 2010 at 12:14 pm

Your system is very nice. But if u get 182*11 what will be the system for calculation. pls tell me.Reply

Michael F. Cox / 11 Aug, 2010 at 10:10 pm

I do it a bitdifferently…1 8 2

+ 1 8 2


12 of 24 3/19/2011 1:33 PM

Simple addition.

It's what they taught us is Grade School… using zero as a place holder…10 x 182 = 1 8 2 (0)

+ (1 x 182) = 1 8 2——————

2 0 0 2

Reply

umesh / 23 Aug, 2010 at 09:17 am

ur method is great and useful to anyone . thank u very much. if u have other short-cut tricks of math please sendme

Reply

Paul Joakim / 18 Sep, 2007 at 02:41 amAnother approach is the “is x divisable by y?”-question:

A number is divisable by 2 if it’s an even number.

3: The sum of the individual digits is divisable by 3, e.g. 174 => 1+7+4 = 12 (=> 1+2 = 3)4: The last two digits is divisable by 4

5: The last digit is 5 or 06: The number is divisable by 2 and 3

7: Double the last digit and subtract it from the rest. If the result is divisable by 7, the original number also is. E.g.

483 => 48-(3*2) = 42, divisable by 78: The last three digits is divisable by 8

9: The sum of digits is 910: The last digit is 0

11: Subtract the sum of “even” digits from the sum of “odd” digits – if divisable by 11 (incl 0), the original numberis divisable by 11: E.g. 7084 => (7+8)-(0+4)=11. 7084 is divisable by 11.

Reply

Robert / 18 Sep, 2007 at 07:43 amI always liked the trick to determine if an integer is divisible by 3 – just add up all the digits in the number, and it

those digits are divisible by 3, the original number is divisible by 3. 114 = 1+1+4 = 6 – 114 is divisible by 3.

Reply

Professor Homunculus / 18 Sep, 2007 at 09:32 am

Check out MathMojo.com for other tricks and tips. There is a link to a good, interactive lesson on multplicationby 11 and 12 for huge numbers on the homepage.

Reply

frogandpig / 18 Sep, 2007 at 11:39 amI have always figured out how much a certain percent is by using either 1% or 10%, then easy basic math.

Example:

4% of 430 = ?1% of 430 = 4.3

4.3 x 4 = 17.2or

(4×4)+(.3×4)=16+1.2=17.2


13 of 24 3/19/2011 1:33 PM

15% of 58 = ?

10% of 58 = 5.85% of 58 = 5.8/2 = 2.9

15% of 58 = 5.8 + 2.9 = 8.7or 10% = 5.8 and 1% = .58

.58 x 5 = (.60 x 5) – .1 = 2.9

Rounding works great for tips at restaurants15% of $58 = ?

15% of $60 = 10%($6) + 5%($3) = $9And if you round but still want the exact amount, subtract the percent you’re trying to find from the additional

amount you rounded. (Works easier for large numbers, but the concept is the same.)

60 – 58 = 215% of 2 = .3

9 – .3 = 8.7Decimal places are moved easily in my mind, so maybe this trick only works for me?

Reply

keisha / 20 Feb, 2011 at 02:28 amThanx but these top ten arithmatic tricks did’nt really help me as i got confused half way through a couple of them

so if i was you i would make it easier to UNDERSTAND!Reply

imajoebob / 18 Sep, 2007 at 03:33 pm

Why didn’t you follow your own tax tip rules for multiplying by 15?Instead of multiplying the first factor by 10, and then the second by 5, which some people may find difficult, just

multiply by 10 (i.e. add a zero) then divide by 2 and add it to itself.237 x 15 =

(237×10) + (237*10)/5 =

2370 + 1185 = 3555Or you can always multiply by 100 and use your 15% tax formula.

Reply

Tom H / 18 Sep, 2007 at 03:48 pmA better tip for multiplying by 6: multiply by 5 (as stated) and then add the original number.

I find this easier than multiplying by 3 then 2…Reply

monsolo / 18 Sep, 2007 at 04:22 pm

Here’s a quick tip to multiply any two numbers that are between 11-19 (like 13 * 15):1. Take the first number (13) and multiply by 10

(13*10)=1302. Take the last digit of the second number (5) and multiply by 10

5*10=50

3. Add the results of steps 1 and 2130+50=180

4. Get the last digit of the first number (3) and the last digit of the second number (5) and mutiply the two3*5=15

5. Add the results of step 3 and 4


14 of 24 3/19/2011 1:33 PM

180+15=195

Try it. It sounds complicated but once you remember it, it gets usefulReply

achan / 18 Sep, 2007 at 05:10 pmFound this neat trick about the 11 rule – expanded.

What is 51236 X 11.

First step, 051236Second step, 5(5+1)(1+2)(2+3)(3+6)6

Answer….563596!Found it here

http://www.angelfire.com/me/marmalade/mathtips.html

Reply

Ace / 18 Sep, 2007 at 07:09 pm

Awesome tips guys! Also.. Monsolo, I like yours too. Thanks!

Reply

sverrirp / 18 Sep, 2007 at 07:47 pm

Trick number two doesn’t work out:35*35 = (2x(3+1)) & 25

2 x 4 = 8 = 825 according to the trick

But 35*35 = 1225Reply

HunterDad / 6 Oct, 2010 at 06:43 amYou need to read the trick better. MULTIPLY THE FIRST NUMBER BY ITSELF PLUS ONE.

35*35 = (3X(3=1)) & 25

3×4 = 12 & 251225.

Reply

Jim C. / 18 Sep, 2007 at 07:53 pmTo convert between ounces and pounds, divide or multiply (as required) by 2 four times.

The same applies for price per ounce and price per pound.Reply

hypergene / 18 Sep, 2007 at 08:10 pm

@sverripTrick 2 is correct:

35*35 = 3 x (3+1) & 25 = 12 & 25 = 122545*45 = 4 x (4+1) & 25 = 20 & 25 = 2025

55*55 = 5 x 6 & 25 = 30 & 25 = 3025

…Reply


15 of 24 3/19/2011 1:33 PM

onikirol / 18 Sep, 2007 at 09:02 pm

even numbers multiplied by 6:6×2= 12

6×4= 246×6= 36

6×8= 48

follow this pattern:the even number is placed as the 2nd digit, and half of it is placed as the first. (the first step always applies, but the

2nd step is only true for 2, 4, 6, and 8 )and for multiples of 5, I’m like Miss Cellania in that I go for multiply by 10 and then half it

Reply

MikeB / 18 Sep, 2007 at 09:43 pmYou might want to look at this book: “The Trachtenberg Speed System of Basic Mathematics” which explains for

example how to multiply two large numbers without the need for calculator or paper.

Reply


imajoebob: you are correct, but I wanted to mention as many options as possible because of the very reason you

state – some people find different methods easier – two methods are better than one Thanks guys for the extra tips – they are all really useful and make for an excellent resource.

Reply

Colin Seymour / 19 Sep, 2007 at 02:39 am

Fantastic list. I use a fair few of these already, but have certainly learnt a few more. Trick 9 applies to 10 to the

power of any number – 10, 100, 1000, 10000 etc too.Reply


Colin Seymour: thanks I am glad it will be of use to you – thanks also for expanding on trick 9. These

comments are as useful as the list

Reply

Rui Martins / 19 Sep, 2007 at 03:23 amActually, Rule Number 3 (Multiply by 5) is a complicated form, of explaining what already is said in in First Item

in Rule Number 10 (Multiply by 5: Multiply by 10 and divide by 2)And this is very simple to understand because, 10/2 = 5

so,

2682 x 5 = 2682 x (10/2) = 26820 / 2 = 13410or if you prefer to halve first

2682 x (10/2) = (2682/2) * 10 = 1341 * 10 = 13410

No weird rules for 0 or 5 just plain multiplication and halving

Remember that multiply by 10 in decimal (base 10 system) is easy, just add 0 to the right.

And halving something is second nature to any one which had a brother Reply


16 of 24 3/19/2011 1:33 PM

Tuplad / 19 Sep, 2007 at 04:38 am

This is great, I suck at math so this is very handy!!Reply

Woody / 19 Sep, 2007 at 05:09 am

One trick I learned thats useful is how to do square roots when you don’t have a calculator handy. It’s simple andworks quite nicely as follows:

Subtract successive odd numbers, starting at 1, until your remainder is less than the next odd number.Count the number of odds you subtracted, thats your whole part, then put the next put your remaining value over

the next odd to get your fractional part.

The fractional part isn’t exact, but it’s usually close for large numbers.For example:

9 -1 = 8 -3 = 5 -5 = 01 2 3

So the square of 9 = 3 (count of 1,3,5)

You can also use addition to get there as well:120 = (1+3+5+7+9+11+13+15+17+19)+20

1 2 3 4 5 6 7 8 9 10 remainderSo, the square of 120 is 10 and 20/21 (10.9523)

The square of 120 is actually 10.9544, but 20/21 is pretty close and easier to work with if you’re not using a

calculator.If you have a rough idea of where its at, you can also figure out a quick “jump in” by guessing close. The close

value can be multiplied by 2, and added to 1 to get the next odd in the sequence. For example, lets do 10535:For 10535 we know 100^2 is 10000.

(100 * 2)+1 = 201, the next odd to use.535 – 201= 334 -203 = 131

So Sqtr(10535) ~= 102 and 131/205 (.639 vs .640)

Hope this is helpful to someone!Reply


I am amazed at how mathematical our users are here! Well done.Oh – and hi to everyone coming from lifehack.org – thanks for visiting and contributing to the comments!

Reply

Logi Helgu / 19 Sep, 2007 at 06:15 am

http://en.wikipedia.org/wiki/Trachtenberg_system

Reply

portnoy / 19 Sep, 2007 at 10:30 am

Of course any even number is divisible by two but,

If the last two digits of a number are divisible by 4 then the whole number is divisible by 4.If the last three digits are divisible by 8 then the whole number is divisible by 8…. etc.

The rules about 3 apply to 6 and 9 too.If the digits add up to be a multiple of 3 and the number is even then it’s divisible by 6.

If the digits add up to be a multiple of 9 then it’s divisible by 9.


17 of 24 3/19/2011 1:33 PM

Seven is the only goofball.

Reply

HunterDad / 6 Oct, 2010 at 07:31 am

"If the digits add up to be a multiple of 9 then it's divisible by 9"Uh… Isn't any number that is a Multiple of X, also divisible by X?

That is like saying that any number that is a multiple of 5, is also divisible by 5.

Kind of obvious?Reply

Jay / 6 Oct, 2010 at 08:15 am

He's talking about cases where the individual digits ADD UP to a multiple of nine. 3+3+3=9; 333/9=37Reply

swimner / 20 Sep, 2007 at 12:41 amRelations of square numbers

We can get 15^2 = 225

What about 13^2?13^2 = 15^2 – (15+13)(15-13) = 225 – 28*2 = 169

17^2 = 15^2 – (15+17)(15-17) = 225 -(-64) = 225 + 64 = 289Twisting the trick a bit, we can generate ALL Pythagorean Triplets

Most people will recall a 3,4,5 triangle

Some remember the 5,12,13 triangleFew remember the 7,24,25 triangle

Most don’t know the 121,7320,7321 triangleNote that the smallest side is odd.

Simply square an odd number such as 9^2 = 81

Divide in half (not evenly)An uneven split of 81 is 40,41

So 9,40,41 is a pythagorean tripletie 9^2 + 40^2 = 41^2

It also works for even numbers, but they have to have at least one odd factor.

Lets try 10. Find the pythagorean triplet for 5.5,12,13 and then muliply by the other factor of 10 (namely 2) to get 10,24,26.

For powers of 2 we must be really sneaky.Take 2^6 = 64.

Divide the number by 4 to get 16.

We know 3,4,5 is a pythagorean triplet.Multiply the triplet by 16 to get 48,64,80.

We owe this to the Babyloneans (3000+ years old).Reply

rosy / 20 Sep, 2007 at 05:44 am

That pretty excellent and good!! I am really impressed with the way you have come up with the math..I will surelyuse this technique…

Reply


18 of 24 3/19/2011 1:33 PM

cybernezumi / 20 Sep, 2007 at 12:50 pm

For restaurant tips I usually just double the sales tax, which here is about 7.5% — yielding 15%. If yours is a bitlower or higher, just round up or down a little bit to get it close enough.

Reply

Nazzyon / 20 Sep, 2007 at 01:01 pmI am good @ Maths but my problem is speed/time.

Now I will be able to solve faster than usual. My B’s will definitely fire up to A’s if I make use of these tricks.Thanks for contributing

Reply

pallav / 21 Sep, 2007 at 03:38 amyou people are scary!!

Reply


pallav: but they will be earning the big bucks one day (if not already!) Nerds rule

Reply

mamoy / 25 Sep, 2007 at 05:18 amits soooooo amazing

Reply

sahil / 29 Sep, 2007 at 07:22 am

its simply superb!!!!!!!!!!!

Reply

flower / 29 Sep, 2007 at 11:45 pm

Fear of Maths is only mental.I suggest:1.Instead of saying DIVIDE BY 2, say HALF/HALVE IT.

2.Instead of saying MULTIPLY it by 2, say DOUBLE IT.

3.Never use more than two digit numbers to prove the working of a method.4.Show the more interesting sides of maths, for example,show the beauty of the table of nine (which really looks

cute, simple and well arranged).After these small things, leave the person to grow up inside herself, by herself. They’ll start with small victories,

and keep gathering courage for bigger ones. Maths IS easy and beautiful upto a certain level. Let’s all enjoy this

beautiful, universal language.Reply

wellwisher / 4 Jan, 2011 at 03:33 am

I really liked your thought, but this will not help to solve tough problems……..Reply


19 of 24 3/19/2011 1:33 PM

Gaurav Tekriwal / 30 Sep, 2007 at 06:32 am

HI!Isnt all this High Speed Vedic Mental Mathematics from India?

Vedic Math is the world’s fastest mental math method from India- You can do calculations much much fastercompared to the normal method or even a calculator.

Check out this site on Vedic Maths which has lots of tutorials and resources.

The tutorials given here on Vedic Maths are the best. Do Check them out.

Hope it helps!

Gaurav Tekriwal

http://www.vedicmathsindia.orgReply

wellwisher / 4 Jan, 2011 at 03:35 amThank you for the suggestion I'll try that………..

Reply

Catdancer / 2 Oct, 2007 at 06:13 amGood to know these things! I’m an elementary teacher and have used a few of these before but not all. As for the

finger trick when multiplying by 9, I teach that every year. It’s great for elementary age kids and some higher

grades, too.I have a trick for calculating a tip that doesn’t involve anything tricky. I figure out what 10% of the bill is by

moving a decimal point, then I add the amount of the sales tax to the 10% figure. My state has a 6% sales tax so itworks well for me. If you had 8%, you could just double the amount of the tax. I never worry about an extra 1%

for the tip – that’s usually just a few pennies.Reply

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