15% of 240 is 36. Here 15 is the percentage rate and 240 is the whole. Multiply 240 by 15/100, or by the decimal 0.15, and keep the base visible so this question is not confused with “15 is what percent of 240?”
Write the percentage as a fraction of 100
15% means 15/100, so 240 × 15/100 = 3600/100 = 36. The decimal method is the same operation written as 240 × 0.15 = 36. Dividing the percentage by 100 before multiplying preserves which value is the rate and which is the whole.
Find 10% and 5% mentally
Ten percent of 240 is 24 because dividing by ten moves the decimal one place. Five percent is half of that, 12. Add the two parts: 24 + 12 = 36. This mental method is a useful independent check on the fraction calculation.
Check the answer as part of the whole
Divide the result by the base: 36 ÷ 240 × 100 = 15%. This reverse check confirms both the arithmetic and the chosen base. It would fail if 15 and 240 had been swapped.
A 15% discount gives a different final price
The percentage amount is 36. If 240 is a price and the task asks for 15% off, subtract that amount: 240 − 36 = 204. If it asks for a 15% increase, add it: 240 + 36 = 276. Do not report 204 as “15% of 240”; it is the value remaining after the reduction.