Adding and Subtracting
Decimals
Lesson 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.
commutative
• Then, group the first two addends together:
(52 + 48) + 12.4
________________
property
associative
• 100 + 12.4 = 112.4
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)
28.90
+ 13.31
42.21
The 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
0 14
9
9
10
15 .000
- 9.372
5 .6 28
Homework Time
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