Multiply by 5: halve it, then multiply by 10

The trick in one sentence

Multiplying by 5 is dividing by 2 then multiplying by 10 (or the reverse).

Why it works

$5 = \dfrac{10}{2}$. So $n \times 5 = n \times \dfrac{10}{2} = \dfrac{n \times 10}{2}$.

Examples

• $84 \times 5$ → half of 84 = 42 → $\times 10$ = 420
• $126 \times 5$ → half = 63 → 630
• $37 \times 5$ → half = 18.5 → 185 (the « .5 » vanishes when ×10)

Why it’s faster

Halving is instant in your head, even on large numbers. Doing $\times 5$ by hand means handling carries.