A certain student has taken 124 credit hours so far in college. The student took the same number of credit hours in each fall semester and the same number of credit hours in each spring semester. The equation 15x + 16y 124 represents this situation, where x is the number of fall semesters and y is the number of spring semesters in which the student has taken classes. How many more credit hours did the student take in each spring semester than in each fall semester?