David did w more push-ups than Zachary in gym class today. If David did x push-ups How many push-ups did Zachary and David do altogether?
Let's say that David did x push-ups. 

Zachary did w push-ups. 

David did more push-ups than Zachary, so:

x > w

Now we need to find out how many push-ups David and Zachary did altogether. 

We can use the fact that the sum of two numbers is equal to the difference between the larger number and the smaller number plus the smaller number. 

The sum of two numbers is equal to the difference between the larger number and the smaller number plus the smaller number:

x + w = (x - w) + w

We know that x is larger than w, so we can write:

x + w = (x - w) + w

x + w = x - w + w

x + w = x

Now we know that the sum of David's and Zachary's push-ups is equal to x.