Zachary did 37 push-ups in gym class today. David did 39 more push-ups than Zachary. John did 30 push-ups less than David. How many push-ups did John do?
Zachary did 37 push-ups. 
David did 39 more push-ups than Zachary. 
John did 30 push-ups less than David. 

That means that David did 76 push-ups in total. So if John did 30 push-ups less than David, John must have done 46 push-ups.