What is 10 per cent of 250?

Per cent means out of a hundred, so ten per cent is one tenth, and one tenth of 250 is 25. The quickest method is to move the decimal point one place to the left, turning 250.0 into 25.0. That single trick is worth more than any formula, because most everyday percentage questions can be assembled from it. Twenty per cent is ten per cent doubled, five per cent is ten per cent halved, and fifteen per cent is ten plus five. So a fifteen per cent tip on a 250 bill is 25 plus 12.5, which is 37.5. It also helps to know that percentages are reversible: eight per cent of 50 is the same as fifty per cent of 8, which is 4, and the second version is far easier to do in your head. Reaching for a calculator is often slower than rearranging the question.

Percentage rises and falls do not cancel out: a 10 per cent drop followed by a 10 per cent rise leaves you below where you began.