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 at the park. 
He collected y bottle caps and z wrappers. 
Therefore, he has (w + x) - (y + z) = (w + x) - (w + x) = 0 bottle caps and wrappers in his collection now. 

We can see that the number of bottle caps is equal to the number of wrappers.