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?
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 and wrappers at first. 

We know that he has y bottle caps and z wrappers in his collection now, which means that he found w + x - (y + z) = w - y + x - z bottle caps and wrappers at the park. 

We also know that he found x wrappers at the park, which means that he found w - y + x - z - x = w - y wrappers at the park. 

Therefore, the answer is w - y.