Each week Jon spends 3/4 of his allowance on school lunches. He saves 1/5 of his allowance. How much more of his allowance does he spend on school lunches than he saves?
There are several different ways to look at this question, but the simplest is to simply subtract the fraction of his allowance Jon manges to save from the fraction he spends on lunches. (Why don't his parents pay for his lunch?!)
You can put in a variable
Or you can convert them to percentages of his allowance and subtract those: