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?
Let's say that the number of wrappers Danny had at first was w. 

After he found x wrappers at the park, he had w + x wrappers in total. 

Now he has y bottle caps and z wrappers in his collection, so we can write: 

y + z = w + x 

z = w + x - y 

Thus, the number of wrappers Danny had at first was w = z + y - x.