EXPLANATION
First you will need TWO numbers. One being the percentage and the other being the number your trying to find the percent out of.
EXAMPLE
Imagine you have 130 crayons. Out of the 130 crayons you found that 15% of them were broken.
First you want too take the 15% and put it over 100, because a percentage always goes over 100. Now that you have the 15/100 you now wanna take the 130 crayons and put the variable X over it. Now that you have the fractions 15/100 and X/130 you want to cross multiply them to get 100x and 1950. For the final step your going to want to take 1950 and divide it by 100 to get your answer, 19.5.
I hope this helped you find out the method for solving a percentage problem while using proportions.