$15 would be 1/5 (80%) of an uknown number, which is stated in the question. Since it's 1/5, you would multiply 15 by 5, to get 5/5 (100%), which is the original price. To check this, you would take 80% percent from 75 (0.8*75), which gives you 15. Thus proving $75 dollars was the original price.