The condition says N*P = 120 (1) (N+2)*(P-2) = 120 (2) where N = the number of students, P = how much each of N students must pay. Transform (2) in this way: N*P + 2P - 2N - 4 = 120, 120 + 2(P-N) - 4 = 120, 2*(P-N) = 4 ====> P-N = 2. So, you have PN = 120, P-N = 2, and you can easily (MENTALLY) get the answer P = 12, N = 10.