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?
First, we need to find out how many push-ups and crunches Zachary did in total. We know that Zachary did w push-ups and x crunches, so his total number of exercises must be w + x. 

Now, we need to find out how many push-ups and crunches David did. We know that David did y more push-ups than Zachary, and we know that David did z less crunches than Zachary. This means that David must have done w + y push-ups and x - z crunches. 

Now, we need to find out how many more crunches than push-ups Zachary did. To do this, we need to subtract the number of push-ups from the number of crunches. We know that Zachary did w push-ups and x crunches, so his total number of crunches must be x - w. 

This means that Zachary did (x - w) more crunches than push-ups.

Therefore, the answer (symbolic expression) is: 

x - w