David did 51 push-ups in gym class today. David did 49 more push-ups than Zachary. How many push-ups did Zachary and David do altogether?
David did 51 push-ups. 
Zachary did 49 more push-ups than David. 

That means that Zachary must have done 100 push-ups. 

David and Zachary did a total of 151 push-ups.