So we know that there are 8 slices per large pizza (7), so a total of 56 slices we would set up a proportion 5/8=x/56 8 times 7 gives you 56, and we would do that same thing to the 5 (multiply it by 7) to get 35? so now we know that 35 out of 56 slices were eaten 56-35 is 21 so there are 21 slices left over
