Answer:
15
Step-by-step explanation:
Michael has 6 unique books, but 4 not-unique slots. Since the slot is not unique, the orders you pick the 4 books are not important. When order important you use permutation but since the order is not important you use the combination. The calculation will be:
6C4 = [tex]\frac{6!}{6!(6-4)!}[/tex] = 15
There are 15 ways to group the 4 books.
