David did 16 push-ups in gym class today. David did 18 more push-ups than Zachary. How many push-ups did Zachary and David do altogether?
David did 16 push-ups in gym class today. 

That means that Zachary must have done 2 less than David, or 16 - 2 = 14. 

Now we can add those two numbers together to get our answer: 

16 + 14 = 30