Adding and Subtracting Decimals

Show your work for each problem.

1.
What is $2.3 + 1.5$ ?
1. a)4.8
2. b)3.7
3. c)3.8
4. d)2.8
Answer: 3.8
$2.3 + 1.5 = 3.8$ . Add tenths: $3 + 5 = 8$ . Add ones: $2 + 1 = 3$ . Result: $3.8$
2.
Calculate $5.7 - 2.4$
Answer: 3.3
$5.7 - 2.4 = 3.3$ . Tenths: $7 - 4 = 3$ . Ones: $5 - 2 = 3$ . Don't forget the decimal point!
3.
What is $4.25 + 3.14$ ?
1. a)7.29
2. b)7.39
3. c)8.39
4. d)7.49
Answer: 7.39
$4.25 + 3.14 = 7.39$ . Work from right to left: hundredths ( $5+4=9$ ), tenths ( $2+1=3$ ), ones ( $4+3=7$ ).
4.
Calculate $8.6 + 1.3$
Answer: 9.9
$8.6 + 1.3 = 9.9$ . Tenths: $6 + 3 = 9$ . Ones: $8 + 1 = 9$ . Result: $9.9$
5.
Calculate $6.45 - 2.23$
Answer: 4.22
1. Subtract the hundredths: $5 - 3 = ?$ 2
2. Subtract the tenths: $4 - 2 = ?$ 2
3. Subtract the ones: $6 - 2 = ?$ 4
4. What is the final answer? 4.22
6.
What is $7.3 + 2.85$ ?
1. a)10.05
2. b)10.15
3. c)9.15
4. d)9.85
Answer: 10.15
$7.30 + 2.85 = 10.15$ . Hundredths: $0+5=5$ . Tenths: $3+8=11$ (write 1, carry 1). Ones: $7+2+1=10$ .
7.
Calculate $9.2 - 4.75$
Answer: 4.45
$9.20 - 4.75 = 4.45$ . Borrowing is needed: $10-5=5$ hundredths, $1-7$ borrows to get $11-7=4$ tenths, $8-4=4$ ones.
8.
Calculate $5.08 + 3.97$
Answer: 9.05
1. Add the hundredths: $8 + 7 = ?$ 15
2. Write 5 and carry 1 to tenths. Now add tenths: $0 + 9 + 1 = ?$ 10
3. Write 0 and carry 1 to ones. Now add ones: $5 + 3 + 1 = ?$ 9
4. What is the final answer? 9.05
9.
What is $12.4 - 7.68$ ?
1. a)4.72
2. b)5.32
3. c)5.72
4. d)4.82
Answer: 4.72
$12.40 - 7.68 = 4.72$ . Hundredths: $10-8=2$ (borrowed). Tenths: $3-6$ borrows to get $13-6=7$ . Ones: $1-7$ borrows to get $11-7=4$ .
10.
A notebook costs 3.49 dollars and a pen costs 1.75 dollars. What is the total cost?
Answer: 5.24
$3.49 + 1.75 = 5.24$ . Hundredths: $9+5=14$ (write 4, carry 1). Tenths: $4+7+1=12$ (write 2, carry 1). Ones: $3+1+1=5$ .
11.
Calculate $8.03 - 2.58$
Answer: 5.45
1. The tenths digit of $8.03$ is $0$ , so first borrow from the ones: $8$ ones become $7$ and $0$ tenths become $10$ . Then borrow 1 tenth for the hundredths. What is $13 - 8$ ? 5
2. The tenths column now has $9$ (10 minus the tenth you gave away). What is $9 - 5$ ? 4
3. The ones column now has $7$ . What is $7 - 2$ ? 5
4. What is the final answer? 5.45
12.
Calculate $15.2 + 8.374$
Answer: 23.574
$15.200 + 8.374 = 23.574$ . Thousandths: $0+4=4$ . Hundredths: $0+7=7$ . Tenths: $2+3=5$ . Ones: $5+8=13$ (write 3, carry 1). Tens: $1+1=2$ .
13.
A runner's time improved from 54.28 seconds to 51.95 seconds. How much faster did they run?
1. a)3.33 seconds
2. b)2.23 seconds
3. c)2.33 seconds
4. d)2.43 seconds
Answer: 2.33 seconds
$54.28 - 51.95 = 2.33$ seconds. The improvement is the difference between the old (slower) time and the new (faster) time.
14.
Calculate $20 - 13.67$
Answer: 6.33
1. Write $20$ as $20.00$ . How many zeros do you write after the decimal point? 2
2. For hundredths: $0 - 7$ requires borrowing. After borrowing, $10 - 7 = ?$ 3
3. For tenths (after giving 1 to hundredths): $9 - 6 = ?$ 3
4. For ones: $9 - 3 = ?$ (the 0 became 9 after borrowing) 6
5. For tens: $1 - 1 = ?$ 0
6. What is the final answer? 6.33
15.
You have 50 dollars. You spend 18.75 dollars on a book and 12.49 dollars on lunch. How much do you have left?
Answer: 18.76
1. First, find the total spent. Add $18.75 + 12.49$ . What is the sum? 31.24
2. Now subtract from 50. Write 50 as $50.00$ . Calculate $50.00 - 31.24$ 18.76