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Numbers5 min read

Mental math tricks that make sums easier

Five ways to do arithmetic in your head: rounding and adjusting, splitting numbers, doubling and halving, working left to right, and checking with estimates.

Wits teamPublished

Round, then adjust

Numbers close to a round number are easier to handle if you round first and fix the difference after. To work out 29 × 4, do 30 × 4 = 120, then take away the extra 4: 116.

The same works for adding. 198 + 57 is 200 + 57 = 257, minus 2: 255.

Split numbers into parts

Break a number into tens and ones and handle each part. 47 + 36 is 40 + 30 = 70 and 7 + 6 = 13, so 83. For multiplying, 23 × 6 is 20 × 6 = 120 plus 3 × 6 = 18: 138.

Double and halve

When you multiply, you can halve one number and double the other without changing the answer. 16 × 25 becomes 8 × 50, then 4 × 100: 400.

Work left to right

At school, sums are often taught right to left on paper. In your head it’s usually easier to start with the biggest part. For 352 + 417, add the hundreds (700), then the tens (60), then the ones (9): 769.

Check with a quick estimate

Key idea

Before you commit to an answer, ask whether it’s the right size. 6 × 48 should be a little under 6 × 50 = 300, so 288 looks right and 2,880 doesn’t.

Practice a little, often

Short, regular practice keeps these steps quick. Speed Math, a free Wits game, starts with sums up to 20 and adds multiplying, dividing, chains and two-step sums as you clear levels.

  1. Level 1: adding and taking away
  2. Level 3: multiplying joins
  3. Level 5: dividing joins
  4. Level 8: two-step sums like 6 × 4 + 5

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