Four percentage questions cover almost everything people actually need: what is X percent of a number, what percentage is one number of another, what is the percentage change between two numbers, and what number does a known percentage come from. All four are below, each showing the formula it used so you can check the working or reuse it.
Divide the percentage by 100 and multiply by the number. 15% of 240 is 0.15 x 240 = 36.
Percentage change is relative to the starting value: going from 4% to 5% is a 25% increase. Percentage points are the absolute gap: the same move is a rise of 1 percentage point. Interest rates and poll results are usually quoted in percentage points.
Each percentage is taken from a different base. 100 plus 10% is 110, and 10% of 110 is 11, so you land on 99. To reverse a 10% increase, divide by 1.1 rather than subtracting 10%.