Mental math tricks that never fail

A little mental math goes a long way. Here are five reliable shortcuts worth internalizing.

Multiply by 5

Multiplying by 5 is the same as multiplying by 10 and halving. 84 × 5 = 840 / 2 = 420.

Multiply by 11

For two-digit numbers, add the two digits and drop the sum in the middle. 45 × 11 = 4 (4+5) 5 = 495.

Round and correct

Turn hard addition into easy addition plus a small correction. 197 + 348 → 200 + 348 − 3 = 545.

Squaring numbers ending in 5

Take the leading digit, multiply by (itself + 1), then append 25. 35² = 3×4 = 12, then 25 → 1225.

Halve then halve

Dividing by 4 is halving twice. Dividing by 8 is halving three times. Chain them together for surprisingly fast division.