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?
Let's denote the number of push-ups Zachary did by x. Then the number of push-ups David did is x+49. 

The total number of push-ups is 

x+x+49=2x+49. 

Therefore the answer (numeric) is: 

2x+49 = 2*51+49 = 153