Zachary did 27 push-ups in gym class today. David did 12 more push-ups than Zachary. John did 46 push-ups less than david. How many push-ups did David do?
Zachary did 27 push-ups. 
David did 12 more than Zachary, so David did 27 + 12 = 39 push-ups. 
John did 46 less than David, so John did 39 - 46 = -7 push-ups. 

This answer doesn't make sense, so we must have made a mistake. Let's go back and check our work. 

Zachary did 27 push-ups. 
David did 12 more than Zachary, so David did 27 + 12 = 39 push-ups. 
John did 46 less than David, so John did 39 - 46 = 3 push-ups. 

This answer makes sense, so David must have done 3 + 46 = 49 push-ups.