If Max, bought a large pizza, and we are assuming it's a whole pizza, think of the pizza as having 8 slices. So when he first buys the pizza, he has all of the slices or 8/8 slices. If he ate 5 of the slices, 8-5=3 so 3 slices are left. Put the denominators into the equation to get
8/8-5/8=3/8
3/8 of the pizza is left. This is a better answer than 3 slices because we don't know for sure that there were originally 8 slices.
Hope this helped!