What does P(n,5) mean?

a) is correct

b) is correct

Well, and 4! means, that you have this event:

Mathbooks, Book A, Book B, Book C, Book D

Mathbooks, Book A, Book C, Book B, Book D

...

What about

Book A, Book A, Book C, Book B, Book D?

There are

Mathbooks, A,B,C,D

A,Mathbooks,B,C,D

A, B, mathbooks, C, D

A,B, C, mathbooks, D

A,B,C,D, mathbooks

5 different ways, thats why I think the soluition is 4!*5