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?
Zachary did w push-ups and x crunches. 
David did y more push-ups than Zachary, so he did w+y push-ups. 
David did z less crunches than Zachary, so he did x-z crunches. 

Now we can compare the two. 
Zachary did w push-ups and x crunches. 
David did w+y push-ups and x-z crunches. 

Zachary did more crunches than David, so x>x-z. 
That means z>0. 

Zachary did x crunches and w+y push-ups. 
David did x-z crunches and w+y push-ups. 

Zachary did more push-ups than David, so w+y>x-z. 
That means w+y+z>x. 

We can add w to both sides. 
w+w+y+z>x+w 
2w+y+z>x+w 
y+z>x 

Zachary