David did 56 more push-ups than Zachary in gym class today. If David did 38 push-ups how many push-ups did Zachary and David do altogether?
Let's say that $x$ is the number of push-ups that Zachary did. Then we know that 
$$x + 56 = 38 + 56 \Rightarrow x = 94$$

So, Zachary did 94 push-ups and David did 38 + 56 = 94 push-ups. In total, they did 94 + 94 = 188 push-ups.