Multiplying & dividing fractions & decimals
Multiplying decimals by whole numbers
To multiply a number with 1 decimal place by a whole number, you can use repeated addition, or multiply the whole numbers and then divide by 10.
Two methods
• Repeated addition: count on in steps of the decimal. • Times-and-divide: multiply as if there were no decimal point, then divide by 10 to put it back. Estimate first to check your answer.
Worked examples
Repeated addition
- Work out 4 × 0.3.
- Count in 0.3s: 0.3, 0.6, 0.9, 1.2.
Answer: 4 × 0.3 = 1.2
Times-and-divide (larger numbers)
- Work out 12.9 × 6. Estimate: 13 × 6 ≈ 78.
- Multiply without the point: 129 × 6 = 774.
- Divide by 10 to put the point back: 774 ÷ 10 = 77.4.
Answer: 12.9 × 6 = 77.4
Where you see this in real life
- Working out the cost of several items that each cost a decimal price.
- Finding total distance from a repeated decimal length.
- Scaling up a recipe measured in decimal amounts.
Key words
- Decimal
- A number that uses a decimal point to show parts of a whole.
- Decimal place
- The number of digits to the right of the decimal point.
- Estimate
- A sensible guess of about how big an answer should be.