Whenever you subtract a negative, such as your example, 24 3/8 - - 54 1/8, you must change your negative into a positive. So the problem becomes 24 3/8 + 54 1/8. Add the mixed fractions together and you get 78 4/8. Don' forget to simplify! The answer will become 78 1/2. Don't kill me if I'm wrong!