(a) There are 13 European cities that Greg would eventually like to visit. On his next vacation, though, he only has time to visit 4 of the cities: one on Monday, one on Tuesday, one on Wednesday, and one on Thursday. He is now trying to make a schedule of which city he'll visit on which day. How many different schedules are possible? (Assume that he will not visit a city more than once.)
(b) From the 13 albums released by a musician, the recording company wishes to release 10 in a boxed set. How many different boxed sets are possible?