Is/of=x/100 cross multiply. Look to what word is in front of the number in the word problem. Plug in x for is because you dont know the #. Then put 20 in for of because its of 20. And put 15 in for the percent part, over the 100. Cross multiply, 20*15=x*100... 300=100x divide both sides by 100 & u get 3 as x. & what i do to check it, is divide 3 by 20, and you get 0.15. Which of course turns into 15%