Zachary did w push-ups and x crunches in gym class today. David did y more push-ups but z less crunches than zachary. How many more crunches than push-ups did Zachary do?
Let's denote the number of push-ups by P and the number of crunches by C.

P = w

C = x

Now we can write that Zachary did P push-ups and C crunches.

David did y more push-ups than Zachary, so we can write that:

P' = P + y

David did z less crunches than Zachary, so we can write that:

C' = C - z

Now we can write that David did P' push-ups and C' crunches.

We need to find out how many more crunches than push-ups Zachary did. In other words, we need to find out how many more crunches than push-ups C is greater than P by. We can write this as:

C - P

Plugging in for P and C we get:

x - w