Alternatively: 15% of 100 is clearly 15 15% of 10 is clearly 1.5 15$ of 5 is half of 15% of 10, namely .75 So 17.25 http://arscalcula.com/

Another method is to remember that percentages are symmetric: x% of y is y% of x. This little-known but trivial fact allows you to find some percentages really fast, and this is one of them. 15% of 115 = 115% of 15 = 15 + .15 * 15 = 15 + 1.5^2 = 15 + 2.25 = 17.25.