Divide by 5: double it, then divide by 10

The trick in one sentence

Dividing by 5 is multiplying by 2 then dividing by 10.

Why

$\dfrac{1}{5} = \dfrac{2}{10}$. So $\dfrac{n}{5} = \dfrac{2n}{10}$.

Examples

• $85 \div 5$ → double = 170 → $\div 10$ = 17
• $340 \div 5$ → double = 680 → 68
• $22 \div 5$ → double = 44 → 4.4

The reverse works for ÷25 and ÷50

• Dividing by 25 = multiplying by 4 then dividing by 100.
• Dividing by 50 = multiplying by 2 then dividing by 100.