Multiply, Then Shift

Some divisors are easier in reverse. To divide by 5, double the number and divide by 10: 60 ÷ 5 → 120 ÷ 10 = 12. Doubling and dropping a zero is quicker than dividing by 5 directly.

The same idea scales up. To divide by 25, multiply by 4 and divide by 100; to divide by 50, double and divide by 100. Each turns an awkward divisor into an easy multiply followed by a digit shift.