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Pre-Algebra

Operations with Decimals

Lesson

Decimal arithmetic looks like whole-number arithmetic with one extra rule for each operation: where the decimal point goes.

Adding and subtracting

Line up the decimal points, then add or subtract as usual.

3.7+2.45=6.153.7 + 2.45 = 6.15

Multiplying

Multiply the numbers ignoring decimals. Then count the total decimal places in the original numbers, and put that many in your answer.

0.4×0.3=0.120.4 \times 0.3 = 0.12

(One decimal place + one decimal place = two in the answer.)

Dividing

If the divisor (the number you’re dividing by) has a decimal, slide its decimal to the end and slide the dividend by the same number of places. Then divide as whole numbers.

6.4÷0.8=64÷8=86.4 \div 0.8 = 64 \div 8 = 8

How to type your answer

Just type the number. Drop trailing zeros after the decimal — 2.5, not 2.50. If the answer is a whole number, no decimal point needed.

Practice

Work through these. Stuck? Click Get a hint.

Warm-Up

Quick problems to get going.

Problem 1

0.5+0.30.5 + 0.3

Problem 2

1.2−0.71.2 - 0.7

Problem 3

0.2×0.40.2 \times 0.4

Problem 4

1.5÷0.51.5 \div 0.5

Practice

Standard problems matching the lesson.

Problem 5

3.70+2.453.70 + 2.45

Problem 6

5.6−1.85.6 - 1.8

Problem 7

12.5+7.812.5 + 7.8

Problem 8

9.0−4.279.0 - 4.27

Problem 9

0.4×0.30.4 \times 0.3

Problem 10

2.50×42.50 \times 4

Problem 11

1.2×0.51.2 \times 0.5

Problem 12

6.4÷0.86.4 \div 0.8

Problem 13

8.1÷0.98.1 \div 0.9

Problem 14

0.6÷0.20.6 \div 0.2

Problem 15

4.25+3.604.25 + 3.60

Problem 16

10−2.7010 - 2.70

Problem 17

3×4.253 \times 4.25

Problem 18

4.20×54.20 \times 5

Challenge

Harder problems — edge cases, trickier numbers, multiple steps.

Problem 19

0.05×0.40.05 \times 0.4

Problem 20

1.25×0.081.25 \times 0.08

Problem 21

12.6÷0.612.6 \div 0.6

Problem 22

3.5−4.83.5 - 4.8

Problem 23

0.45÷0.050.45 \div 0.05

Problem 24

2.30+4.75−1.052.30 + 4.75 - 1.05

Problem 25

250.50+75.25−40.30250.50 + 75.25 - 40.30

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