You take the smallest number and see how many times it can fit into the largest . Like 21/3 you see how many times 3 goes into 21 which happens to be 7 . Then it the smallest number would become a 1 and the top is the number that you got so it would be 7/1 which is equal to 7. 5/15 so you would see how many times 5 can go into 15. The 5 becomes a 1 and the 15 becomes a 3 because 5•3 is 15. So it would reduce to 1/5. Hope that's helps if not message me :)