20 is 15 percent of what number?

20 is 15% of 133.33 (actually 133 1/3).

To do this sort of problem, you first convert the percentage to a decimal by dividing it by 100 (in this case 15 / 100 = 0.15) and then you divide the number given by that decimal:

20 / 0.15 = 133.333333...