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Problem
Multiplying large numbers by 5 in your head can be slow, especially with odd numbers.
Solution
Divide the number by 2, then multiply by 10. If the number is odd, divide by 2 and you will get a .5 — just move the decimal.
Benefit
Dividing by 2 is always easier than multiplying by 5 — works for any number, any size.
Example
Even numbers:
48 × 5 → 48 ÷ 2 = 24 → 240 ✓
126 × 5 → 126 ÷ 2 = 63 → 630 ✓
Odd numbers:
37 × 5 → 37 ÷ 2 = 18.5 → 185 ✓
73 × 5 → 73 ÷ 2 = 36.5 → 365 ✓
Why it works: × 5 = × 10 ÷ 2 = ÷ 2 × 10