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 = 6.15

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 \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 \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 + 0.3

Problem 2

1.2 - 0.7

Problem 3

0.2 \times 0.4

Problem 4

1.5 \div 0.5

Practice

Standard problems matching the lesson.

Problem 5

3.7 + 2.45

Problem 6

5.6 - 1.8

Problem 7

12.5 + 7.8

Problem 8

9.0 - 4.27

Problem 9

0.4 \times 0.3

Problem 10

2.5 \times 4

Problem 11

1.2 \times 0.5

Problem 12

6.4 \div 0.8

Problem 13

8.1 \div 0.9

Problem 14

0.6 \div 0.2

Problem 15

4.25 + 3.6

Problem 16

10 - 2.7

Challenge

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

Problem 17

0.05 \times 0.4

Problem 18

1.25 \times 0.08

Problem 19

12.6 \div 0.6

Problem 20

3.5 - 4.8

Problem 21

0.45 \div 0.05

Problem 22

2.3 + 4.75 - 1.05