Operations with Decimals!

Adding and Subtracting DecimalsHow is adding and subtracting decimals similar to adding and subtracting whole

numbers?

Activities 1-3 – See handout

• Is there a relationship between adding and subtracting decimals and adding and subtracting whole numbers?

• When in the real world would you add and subtract decimals?

Steps for adding and subtracting

• Write the numbers down vertically.• Line up the decimal points. • Bring the decimal point straight down.• Add and subtract as you would whole

numbers.

Examples

• Add 8.13 and 2.76. 8.13 +2.76

• Add 1.459 + 23.7 1.459 + 23.7___

Examples

• Subtract 5.508 – 3.174.

5.508 - 3.174

• Subtract 21.9 – 1.605. 21.900 - 1.605

Real – Life Application

$3.45 $10.00+$3.90 - ______

Your turn

• Complete with your clock partner and then get your answers checked by me.

• 4.206 + 10.85 =

• 27.9 – 0.905 =

Conclusion

• How is adding and subtracting decimals similar to adding and subtracting whole numbers?

Multiplying Decimals

How is multiplying decimals similar to and different from multiplying whole

numbers?

Activity – Multiplying Decimals by Decimals

• Complete Activity (pages 7 and 8)• What conjecture can you make about the

relationships between multiplying decimals and multiplying whole numbers?

Steps to Multiplying Decimals

• Multiply as you would with whole numbers. • Count the number of decimal places in the

decimal factor. • The product has the same number of decimal

places.

Examples

• It is helpful to estimate to see if your answer is reasonable!

• Find 6 x 3.91

• Find 3 x 0.0016.

Examples

• Multiply 4.8 x 7.2.

• Multiply 3.1 x 0.05.

Real Life Application

• You buy 2.5 pounds of tomatoes. You hand the cashier a $10 bill. How much change should you receive?

Conclusion

• How is multiplying decimals similar to and different from multiplying whole numbers?

Dividing Decimals

How can you model decimal division with pattern blocks and numbers?

Complete Activity 1 on Division Modeling

• Dividing decimals by whole numbers– Place the decimal point in the quotient above the

decimal point in the dividend.– Then divide as you would with whole numbers.

– Find 7.6 ÷ 4 Find 36.4 ÷ 2

Dividing Decimals by Decimals

• Multiply the divisor and dividend by a power of ten to make the divisor a whole number.

• Place the decimal point in the quotient and divide as you would with whole numbers.

Examples

• Find 18.2 ÷ 1.4.

• Find 9.6 ÷ 1.2

• Find 0.459 ÷ 0.51

Real – World Example

Summary

• How can you model decimal division with pattern blocks and numbers?

• How else can you divide if you don’t draw or use a model?