The first pizza is 6 slices, so each slice is 1/6 of a pizza. The second pizza is 4 slices, so each slice is 1/4 of a pizza. If you were to take one slice of each, you would get 1/6 +1/4 slice, which would be 5/12 of a pizza, as opposed to if you took 2 slices of the first pizza, which would only be 4/12 of a pizza.