Zachary did 18 push-ups in gym class today. David did 41 more push-ups than Zachary. John did 45 push-ups less than David. How many push-ups did John do?
Zachary did 18 push-ups. 
David did 41 more push-ups than Zachary. 
John did 45 push-ups less than David. 

We can use this information to write an equation: 

Zachary + David + John = 18 + 41 + John

We can solve for John: 

John = 18 + 41 - 45 

John = 14