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?
We know:
w bottle caps and x wrappers at the park. Now he has y bottle caps and z wrappers in his collection.

We want to know:
How many wrappers did Danny have at first?

We don't know, but we can find out:
y = w + x
z = y - w

We also know that:
z = x - w

Now we have two equations and two unknowns. We can solve it:

y = w + x
z = y - w

x = w + z

Danny had x wrappers at first.