Mental Math Tricks
Fast mental arithmetic isn't about being "a maths person" — it's a handful of shortcuts that turn hard-looking sums into easy ones. Here are the most useful, each with a worked example. The trick is knowing why each one works, so you remember it.
Multiply by 11
For a two-digit number, add its digits and drop the sum in the middle. 36 × 11 → 3 (3+6) 6 → 396. If the middle sum is 10 or more, carry the 1. It works because ×11 is ×10 + ×1.
Square any number ending in 5
Take the tens digit, multiply it by the next number up, and stick 25 on the end. 65² → 6 × 7 = 42 → 4225. 85² → 8 × 9 = 72 → 7225.
The percentage flip
x% of y equals y% of x — and one side is often much easier. 4% of 25 is hard; flip it to 25% of 4 = 1. 18% of 50 → 50% of 18 = 9.
Multiply by 5
Halve the number, then multiply by 10 (or ×10 then halve for odd numbers). 5 × 48 → half of 48 is 24 → 240.
Add by rounding, then adjust
Round to a friendly number and correct at the end. 67 + 28 → 67 + 30 = 97, then −2 → 95.
Doubling and halving
To multiply, double one number and halve the other to reach an easy product. 14 × 5 → 7 × 10 = 70.
Learn the "why", then practice
Math Challenge has 36 illustrated Magic Tricks lessons that each explain the idea behind the shortcut, then let you practice it — plus a free Daily Challenge and 35 arithmetic topics.