Paul got a box of w erasers and x crayons for his birthday. At the end of the school year he only had y crayons left while not having lost a single eraser. How many more crayons than erasers did he have left?
Let's say that Paul had c crayons at the beginning of the school year. Then he had c - y crayons left at the end of the school year. 

Now we know that he had w + x crayons at the beginning of the school year and c - y crayons at the end of the school year. 

We also know that he had w erasers at the beginning of the school year and w erasers at the end of the school year. 

This means that he had x - y more crayons than erasers at the end of the school year.