Question

1. find the # of ways to display 4 books on a shelf if you have 15.
2. how many ways can you select a committee of 8 people from a group of 13?

Answer 1

1.

15*14*13*12 = 15!/11!=32760

2.

13*12*11*10*9*8*7*6=13!/5!=51,891,840

Answer 2

if the order matters, use permutation:
P(n, r) = n!/(n-r)!
if the order doesn't matter, use combination:
C(n, r) = n!/((n-r)!*r!)

Answer 3

thank you :)

Answer 4

If the order of display of books matters, then it would be 15P4 for the first part....
For second part it should be 13C8...