i’ll express this in model format. after Melvin gives Heping 20 stamps, Heping has 5 times more stamps as Melvin, so the total 84 stamps can be split into 6 units. so i can find the stamps Melvin has after he gave 20 to Heping, so i can add 20 to the final stamps of Melvin to find how many he had at first. then, i can just use 84 minus the number of stamps Melvin had at first to find number of stamps Heping had at first. i hope you understand!