Zachary did 46 push-ups and 58 crunches in gym class today. David did 38 more push-ups but 62 less crunches than zachary. How many more crunches than push-ups did Zachary do?
Let's denote Zachary's number of push-ups by p and his number of crunches by c. Then we have: 

p+c=46 
p-c=58

David's number of push-ups is p+38 and his number of crunches is c-62. 

Therefore we have: 

(p+38)+(c-62)=46 
(p+38)-(c-62)=58

We can solve this system of linear equations and get: 

p=27 
c=35

Zachary did 35 crunches and 27 push-ups. Therefore he did 8 more crunches than push-ups.