Mental math tricks: five easy ways to calculate in your head

To calculate in your head, turn a hard sum into easy ones: round to a friendly number, split a number into tens and ones, or double one side and halve the other. These five habits cover most of what you will meet. None of them needs speed, only a little practice.

1. Round and adjust

To add a number that is close to a round one, add the round number and then take the difference back.

47 + 38: round 38 up to 40. 47 + 40 = 87, then take off the 2 you added: 85.

2. Break a number apart

To multiply, split the bigger number into tens and ones, multiply each part, then add the two answers.

34 × 6: 30 × 6 = 180 and 4 × 6 = 24. 180 + 24 = 204.

3. Double one side, halve the other

Multiplying by 25 is hard, but multiplying by 50 is easy. If one number is even, halve it and double the other: the answer does not change.

25 × 14: halve 14 to get 7, double 25 to get 50. 50 × 7 = 350.

4. Percentages from 10%

Ten percent is one tenth: move the decimal point. Five percent is half of that. Any other percentage that is a multiple of 5 is built from those two.

15% of 240: 10% is 24, 5% is 12. 24 + 12 = 36.

5. Squares of numbers ending in 5

To square a number that ends in 5, multiply its tens digit by the next number up, then write 25 after the result.

35²: the tens digit is 3, the next number is 4. 3 × 4 = 12, then 25 on the end: 1225.

The levels on Gridarium

Every set has 10 questions, with no countdown and no penalty: a wrong answer shows you the right one. The set of the day is gentle on most days and harder only on 1 day a week.

• Easy: simple adding and taking away, small times tables, halves.
• Medium: bigger numbers, the full times tables, easy percentages.
• Hard: two-digit products, bigger sums, percentages such as 15%, squares.