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 $z$. Then, we know that $z + 49 = 51$, so $z = 2$. 

Therefore, Zachary and David did $2 + 51 = 53$ push-ups altogether.