Multiply with friendly numbers
Many multiplications become easy once you rewrite one number as something simpler to work with.
Multiplying by 25
25 is 100 divided by 4. So 48 × 25 = 48 × 100 ÷ 4 = 4,800 ÷ 4 = 1,200.
Splitting a number
Break one number into easy parts. 7.5 × 12 = (7 × 12) + (0.5 × 12) = 84 + 6 = 90.
Percentages from 10%
Find 10% by moving the decimal point one place left, then build other percentages from it. 5% is half of 10%, and 1% is a tenth of 10%.
Example: 15% of 360 is 10% (36) plus 5% (18), which is 54.
- 10%: move the decimal point one place
- 5%: half of 10%
- 20%: double 10%
- Percentages swap: 8% of 50 equals 50% of 8, which is 4
Divide by simplifying first
Cancel common factors before dividing. 3,600 ÷ 24: divide both by 12 to get 300 ÷ 2 = 150.
When dividing by 5, double the number and divide by 10. 435 ÷ 5 = 870 ÷ 10 = 87.
Estimate before you answer
A quick estimate catches misplaced decimals and wrong operations. Round each number, calculate roughly, and check your answer is in the same range.
Example: 487 × 3.1 is roughly 500 × 3 = 1,500. The exact answer, 1,509.7, sits close to that, so it passes the check. An answer of 150 or 15,000 would fail it straight away.
Working under time pressure
Speed comes from recognising which method to use, so practise choosing the method rather than only drilling answers.
- Learn squares to 15 and the 12 times table by heart
- Read the whole question before calculating anything
- If a sum stalls you, estimate, choose the closest answer and move on
- Review every mistake and note whether it was method, slip or time