Multiply by 5 Quickly

Problem

Multiplying large numbers by 5 requires long multiplication.

Solution

Multiply by 10, then divide by 2. Or divide by 2, then multiply by 10.

Benefit

Converts difficult multiplication into simple division/multiplication by 10.

Example

86 × 5:
86 × 10 = 860
860 ÷ 2 = 430

Or: 86 ÷ 2 = 43
43 × 10 = 430