Name: ____________________________________
Period: _____
Lesson 1-7: Adding and Subtracting Decimals
Date: __________________________
Mrs. Torres
6th Grade
This lesson will help you to find the sum or difference of two amounts of money, you need to
add or subtract.
If you estimate before you add or subtract, you can tell whether your answer is reasonable.
One way to estimate is to round.
Example: Finding Decimal Sums
1) Find 3.026 + 14.7 + 1.38
Step #1: Estimate
3.026 + 14.7 + 1.38
≈3
+ 15 + 1
≈19
Step #2: Line up the decimal points and add:
3.026
14.700
Write zeros so that all of the decimals have the same number of digits to
+ 1.380
the right of the right of the decimal point.
19.106
Using front-end estimation:
In front-end estimation, you estimate by first adding the “front-end digits”. Then you estimate
the sum of the remaining digits. You adjust the sum of the front-end digits as necessary.
Example: Use front-end estimation to estimate the total cost of buying one of every size of
popcorn shown at the left.
Popcorn size
X-Large
Large
Medium
Small
Price ($)
9.08
6.49
3.98
3.47
Step #1: Add the front-end digits:
$9
$6
$3
+ $3
$21
Step #2: Estimate the total of cents:
$0.08
$0.49
$0.98
+ $0.47
$2.02
The total cost will be about: $21 + $2 ≈ $23.
Finding a difference:
When you are going to subtract, you must add zeros, if it’s necessary, and then you can
subtract by taking away and regrouping.
Example: Subtract the following decimals:
46 – 24.28
Step #1: Line up the decimal points:
-
46.00
24.28
Write zeros so that all of the decimals have the same number of digits to
the right of the right of the decimal point.
Step #2: Subtract
-
46.00
24.28
21.72 about 22.
Exercises:
I.
First estimate. Then find each sum.
1) 0.6 + 3.4
2) 6.2 + 0.444
3) 8.001 + 0.77
4) 7 + 11.436 + 3.08
6) 0.33 + 1.11 + 3.2
5) 0.445 + 8.99 + 3
II.
Use front-end estimation
1) $4.89 + $3.97
2) $6.15 + $8.86
3) $14.65 + $27.29 + $63.85
4) $16.81 + $19.94 + $11.49
III. First estimate. Then find each difference.
1) 22.2 – 4.3
2) 8.91 – 6.08
3) 25.36 – 3.7
4) 50.13 – 8.26
5) 1.56 – 0.9
6) 19.35 – 15.6