Fives, 25s & halving

× 5 is just × 10, then halve.

Fives and 25s are everywhere in pricing, discounts and budgets. They’re secretly 10 and 100 in disguise, and those are the easiest numbers there are.

The move

× 5 → × 10, then halve. ÷ 5 → double, then ÷ 10. × 25 → × 100, then ÷ 4. Or halve one side, double the other: 16 × 35 = 8 × 70.

Worked example

48 × 5

  1. 48 × 10 = 480
  2. Half of 480 = 240
  • Halving and doubling keeps the answer the same. Keep going until one side is a round number.