Calculate with decimals
Objective. Add, subtract, multiply and divide decimals accurately.
- 1. Learn
- 2. Worked example
- 3. Practice
- 4. Feedback
- 5. Continue
Learn: the key idea
Calculating with decimals uses the same methods as whole numbers, but the decimal point must be tracked carefully. When adding or subtracting, line up the decimal points in columns before working digit by digit. When multiplying, ignore the decimal points first, multiply as whole numbers, then count the total decimal places in both numbers to place the point correctly, so 0.3×0.4=0.12. When dividing by a decimal, multiply both numbers by a power of 10 to make the divisor a whole number before dividing. Checking the answer's size against an estimate catches misplaced decimal points.
Worked example
Calculate 3.6 ÷ 0.4 using decimal methods.
- 1Multiply both numbers by 10 to clear the decimal in the divisor: 36 ÷ 4.
- 2Divide the whole numbers: 36÷4=9.
- 3Check the division by multiplying back: 9×0.4=3.6.
- 4The decimal calculation gives the same result as the original.
Answer: 9
Concept mastery
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Practice
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Start practisingCommon mistake. Pupils often misplace the decimal point when multiplying, writing 0.6×0.7 as 4.2 instead of 0.42. Count the decimal places in both factors to fix the point correctly.
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