A percentage represents a fraction of 100. To find X% of a number Y, multiply Y by X and divide by 100: Result = (X × Y) / 100. For example, 15% of 200 = (15 × 200) / 100 = 30.
Percentage change measures how much a value has increased or decreased relative to the original value: % Change = [(New − Old) / Old] × 100. A positive result indicates increase; negative indicates decrease.
To find 10% of any number, simply move the decimal point one place left. To find 5%, find 10% and halve it. To find 25%, divide by 4. To find 1%, divide by 100. Combine these to quickly estimate any percentage mentally.