Danny collects bottle caps and wrappers. He found w bottle caps and x wrappers at the park. Now he has y bottle caps and z wrappers in his collection. How many wrappers did danny have at first?
First, Danny went to the park and found w bottle caps and x wrappers. 

Next, he brought them home and added them to his collection. 

That means that, before he went to the park, he had y-w bottle caps and z-x wrappers in his collection. 

We are trying to solve for x, so that means that z-x=z. 

Therefore, x=0.