Study tips · 3 min read
Five Mental Math Shortcuts That Save Real Time
1. Build percentages from 10%
Moving the decimal point one place gives 10%. Halve that for 5%. Add them for 15%. Here that is 24 + 12 = 36.
Check: 15% of 240
- 15% of 240 is 36
Step-by-step working 2 steps
- 1Turn the percent into a decimal
15% = 15 ÷ 100 = 0.15
- 2Multiply by the number
0.15 × 240 = 36
2. Multiply by 5 by halving first
Multiplying by 5 is the same as multiplying by 10 and halving. For 48 × 5, halve 48 to get 24, then multiply by 10: 240.
3. Square a number ending in 5
Multiply the tens digit by the next whole number, then append 25. For 35²: 3 × 4 = 12, so the answer is 1225.
4. Multiply by 9 with a subtraction
9 × 7 is 10 × 7 minus 7, which is 63. The same idea works for 99: multiply by 100, subtract once.
5. Check divisibility by 3 with digits
A number is divisible by 3 if its digits add up to a multiple of 3. For 1,236 the digits sum to 12, so 1,236 is divisible by 3 — and indeed 1,236 ÷ 3 = 412.
Frequently asked questions
How can I practise mental math?
Do two or three short problems a day without a calculator, then check with a tool that shows steps so you can see where an error crept in.
Are shortcuts allowed in exams?
Yes — they are just faster ways to reach the same answer. Show your working if the question asks for it.
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