beginner · 8 min read · easy
Mental Math Tricks Worth Memorising
Most mental math books pad with tricks that take longer than just using a calculator. These 15 are the keepers — each saves real time and works under exam pressure.
Squaring numbers ending in 5
25² = (2·3)·100 + 25 = 625. 75² = (7·8)·100 + 25 = 5625. Pattern: drop the 5, multiply the front by itself+1, append 25.
Multiplying by 11 (two-digit)
32 × 11: add the digits (3+2=5), insert in middle → 352. 47 × 11: 4+7=11, carry the 1 → 517. Works flawlessly for 2-digit numbers.
Percentage flips
8% of 50 = 50% of 8 = 4. Whenever one number is a "nice" percentage like 25, 50, 75, swap it to the other side.
Multiplying by 9, 99, 999
7 × 9 = 7 × 10 − 7 = 63. 47 × 99 = 47 × 100 − 47 = 4700 − 47 = 4653. Always subtract from the round number.
Estimating roots
√50 ≈ 7.07 (between 7² = 49 and 8² = 64, much closer to 49). Memorise √2 ≈ 1.41, √3 ≈ 1.73, √5 ≈ 2.24, √7 ≈ 2.65 — combinations get you most quick estimates.
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