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 Zachary did. We know that Zachary did w push-ups and x crunches, and we know that David did y more push-ups but z less crunches than Zachary. This means that Zachary must have done w + y push-ups. 

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

This means that the answer (symbolic expression) is: 

x + z - w - y