Adding and Subtracting DecimalsLesson 1-5

The Basic Steps:

• Line up the numbers by the decimal point.

• Fill in missing places with zeroes.

• Add or subtract.

Addition Properties

Commutative Property of Addition: Numbers can be added in any order.

Example: 5+3=3+5

Associative Property of Addition: Numbers can be grouped in any way.

Example: (7+3) + 5 = 7 + (3+5)

Identity Property of Addition: The sum of any number and zero is that number.

Example: 18 + 0 = 18

Mental Math• Addition properties can be used to quickly

find sums.• Example: What is 52 + 12.4 + 48?• We can use an addition property to

quickly add this. • First, change the order: 52 + 48 + 12.4

________________ property.• Then, group the first two addends

together: (52 + 48) + 12.4________________ property

• 100 + 12.4 = 112.4

commutative

associative

Why Estimate?

• Before adding your numbers, use one of the estimating strategies you learned to see what the answer should be close to.

• When you add, you can see if your answer is reasonable.

Example: 28.9 + 13.31

This can estimate to 30 + 13, which is 43. (Compatible numbers)

28.913.31+

0

42.21The answer is close to the estimate.

Subtracting

• The addition properties do not work for subtraction.

• Estimate and subtract to find what the answer is close to.

• Subtract to find the real answer.

Subtracting Across Zeroes• If you have several zeroes

in a row, and you need to borrow, go to the first digit that is not zero, and borrow.

• All middle zeroes become 9’s.

• The final zero becomes 10.

Example: 15 – 9.372

159.372-

.0004 9 9 10

826.

0 1

5

Homework Time

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