Mental Math 101: Parent Tips

Carla Kolada—Numeracy Lead

Why is Mental Math Important?

•Estimating total cost

•Calculating tax,

discounts, tips

•Doubling/halving a

recipe

•Splitting cab fare

•Buying fabric, lumber,

paint

•Telling time

Believe it or not, we use Mental Math all the time!

O Mental Math is not about using tricks that don’t make sense, but instead, it’s about being able to work with numbers in a way that makes computations easier.

O In order to do that, children (and adults) need to develop excellent number sense.

O If you understand WHY a strategy works, then it’s much more likely that you’ll be able to remember it. Then, you can use it effectively and in a variety of situations.

Let’s try it out!

47 + 34

Hmm…How could I do this in

my head?

We could try this:

47 + 34= 40 + 30 7 + 4

70 + 11 = 81

Or it might look like this:

47 + 34

Perhaps, like this:

47 + 34

Or maybe even like this:

47 57 67 77 80 81

+10 +10 +10 +3 +1

47 + 34

If I count on from 47, that makes

sense, too!

How about this?

52 x 4

Or, maybe I could multiply it by 2

first.

What if I thought about 52 as 50 and 2 more?

We don’t all think the same way, so it’s okay to use different

strategies.

Front-End Addition:

O Start at the left – not always in the ‘ones’ place:

1 489 + 237 1 000 + 600 + 110 + 16 = 1 726

(less chance of calculation error than re-grouping)  

Compatible Numbers Which numbers “go together” to make it easier to do the Math in my head?

25 + 18 + 75

These are also called “nice numbers” or “friendly numbers” 

?

Compensation – sometimes called the ‘Robin Hood’ strategy

O When adding, take from one and give to the other.

398+ 17

Why not take 2 from the 17 and give it to 398?

It becomes 400 + 15 much easier!

Compensation: Constant Difference

178 – 37 You could subtract 7 from each number – keeping the ‘difference’ between the two numbers the same

171 – 30 is easier to calculate!

You could also have added 3 to each…making it 181 – 40

Same answer…because you kept the ‘difference’ between those two numbers the same.

Multiplying TipsHow can I use parts of numbers to multiply faster…and easier? Split them up: 17 x 5 is the same as 10 x 5 plus 7 x 5 50 + 35 = 85 Double - Double to help with groups of 4 7 x 4 = double 7 (14)….then double 14 (28)

Double Half --When multiplying, double one factor and half the other factor. Would you rather multiply 5 x 18? Or 10 x 9?

Let’s Practice!

And, when you feel like calling for reinforcements, remember, think about

what would make sense!