Answer Keys: Adding & Subtracting Decimals

Student Concept Worksheet Answers

1

7.43

4.60 + 2.83 = 7.43 (add zero to 4.6, then add column by column)

2

5.78

9.50 - 3.72 = 5.78 (add zero to 9.5, regroup from tenths: 10-2=8, 4-7 need to borrow, 14-7=7, 8-3=5)

3

6.62

12.00 - 5.38 = 6.62 (write 12 as 12.00, then subtract with regrouping)

4

$5.25

$3.75 + $1.50 = $5.25 (line up decimal points, add)

Practice Worksheet Answers

Part A: Adding Decimals

#	Problem	Answer	Work
1	3.4 + 5.28	8.68	3.40 + 5.28 = 8.68
2	7.65 + 2.8	10.45	7.65 + 2.80 = 10.45
3	12.5 + 4.375	16.875	12.500 + 4.375 = 16.875
4	0.456 + 3.7	4.156	0.456 + 3.700 = 4.156

Part B: Subtracting Decimals

#	Problem	Answer	Work
5	8.7 - 3.45	5.25	8.70 - 3.45 = 5.25
6	15.2 - 8.67	6.53	15.20 - 8.67 = 6.53 (regroup)
7	10 - 4.375	5.625	10.000 - 4.375 = 5.625
8	6.004 - 2.58	3.424	6.004 - 2.580 = 3.424

Part C: Mixed Practice

#	Problem	Answer
9	9.25 + 3.8	13.05
10	14.6 - 7.82	6.78
11	5.008 + 12.99	17.998

Part D: Word Problems

12

7.95 miles

3.75 + 4.20 = 7.95 miles (Tyler's total running distance)

13

$7.15

$20.00 - $12.85 = $7.15 (Samantha's remaining money)

14

6.125 meters

8.500 - 2.375 = 6.125 meters (remaining ribbon)

Challenge Questions:

#15: Jake's answer of 4.55 is NOT correct. The actual answer is 4.45. Estimation: 7 - 3 = 4, so the answer should be close to 4. Jake's answer of 4.55 is close to 5, which suggests an error. The correct work: 7.30 - 2.85 = 4.45.

#16: Missing number: 6.85. Work: 12.45 - 5.6 = 12.45 - 5.60 = 6.85. Check: 5.6 + 6.85 = 12.45

FAST Practice Quiz Answers

1

B) 10.18

6.40 + 3.78 = 10.18 (line up decimals, add with regrouping)

2

A) 4.67

12.50 - 7.83 = 4.67 (regroup from ones: 10-3=7, 14-8=6, 11-7=4)

3

B) $8.15

$5.75 + $2.40 = $8.15 (add the two prices)

4

Select: 4.3 + 3.2, 10.25 - 2.75, 5.8 + 1.7, and 15 - 7.5

4.3 + 3.2 = 7.5
10.25 - 2.75 = 7.5
5.8 + 1.7 = 7.5
15 - 7.5 = 7.5
9.25 - 1.25 = 8 (NOT 7.5)

5

A) 7.225 meters

15.600 - 8.375 = 7.225 meters (add zeros, subtract with regrouping)

6

C) Yes, because rounding 8.7 to 9 and 3.2 to 3 gives 9 - 3 = 6, and the exact answer 5.5 is close to 6

This answer best explains the estimation process AND confirms the estimate is reasonable by showing the exact answer is close to the estimate.

FAST Quiz Scoring Guide

Score	Interpretation	Recommended Action
6/6	Mastery	Ready for multiplying decimals. Challenge with multi-step problems.
4-5/6	Approaching Mastery	Review missed concepts. Focus on regrouping if subtraction was missed.
2-3/6	Developing	Reteach with emphasis on lining up decimal points. Use graph paper for alignment.
0-1/6	Needs Intervention	Small group instruction. Review whole number addition/subtraction with regrouping first, then extend to decimals.

ANSWER KEYS - Teacher Use Only

Part A: Adding Decimals

Part B: Subtracting Decimals

Part C: Mixed Practice

Part D: Word Problems

Challenge Questions: