Danny collects bottle caps and wrappers. He found w bottle caps and x wrappers at the park. Now he has y wrappers and z bottle caps in his collection. How many more wrappers than bottle caps does danny have now?
Danny has w + x bottle caps and wrappers in total. 

He found x wrappers at the park, which means that he had w + (x - x) = w bottle caps before he went to the park. 

Now he has y + x wrappers and w + x - x = w bottle caps. 

This means that he has y + x - w more wrappers than bottle caps.