Most percentage mistakes are not multiplication mistakes. They happen before the arithmetic, when the wrong number is chosen as the base. The wording of the question tells you which quantity represents the whole, reference, or original value. Name that base explicitly before writing the formula.

For X% of a number, the number is the base

To find 15% of 240, convert 15% to 15/100 and multiply by 240: 240 × 15 ÷ 100 = 36. The percentage describes a share of 240, so 240 is the base. This formula works for rates above 100% and decimal rates as well.

For part as a percentage, the whole is the denominator

If 36 is part of 240, calculate 36 ÷ 240 × 100 = 15%. Reversing the division asks a different question: 240 is 666.666667% of 36. Label the part and whole first, and never allow a zero whole.

For percentage change, the original is the base

A move from 80 to 100 changes by 20. Divide that change by the original 80: (100 − 80) ÷ 80 × 100 = 25%. Dividing by 100 would produce 20%, but that describes the change relative to the new value, not growth from the original.