A student answers 75% of the questions on a math exam correctly. If she answers 15 questions correctly, how many questions are on the exam?

You can set up a proportion:    75% / 100%  =  15 / 

    --->     .75 / 1.00  =  15 / x

    --->     .75 · x  =  15                (multiply both sides by x)

    --->     x  =  15 / .75                (divide both sides by .75)

    --->     x  =  20 questions