Adding and Subtracting
Decimals
Lesson 1-5
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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