We will consider catalogues and leaflets separately.
If only one student is delivering the catalogues, then he needs 480 / 15 = 32 hours. But 8 students each doing one hour of work is equal to 1 student doing 8 hours of work (I feel kinda sorry for that student though). So to minimize the cost, we only hire 1 student to do the 8 hours of work. So in total, we need 32 / 8 = 4 students, each doing 8 hours of work. (Sorry, it's for cost minimization )
Similar logic as above, so if one student is delivering the catalogues, he needs 19 hours. But we can just hire 3 students, 2 of which doing 8 hours of work, and the other one doing 3 hours of work(Really lucky xD).
So in total, we have 3 + 4 = 7 students, finishing all the work in 8 hours. High efficiency! :D