David did 18 more push-ups than Zachary in gym class today. If David did 45 push-ups How many push-ups did Zachary do?
1. David did 18 more push-ups than Zachary. 
2. If David did 45 push-ups, then we can say that 18 + 45 = 63. 
3. Therefore, Zachary did 63 - 18 = 45 push-ups.