Question

The number of ways to listen to 4 different cds from selection of 15 cds

Answer 1

32,760 is the correct answer, the first person is wrong! Took the test and this is it!

Explanation:

Answer 2

In general number of choosing r things from the group of n number of things(When order doesn't matter) =
`^nC_r`

We can expand
`^nC_r` as
`^nC_r = (n!)/(r! * (n-r)!)`

So same way we can find number of way to listen 4 different cds from 15 cds =
`^(15)C_4`

Which we can calculate as

`^(15)C_4 = (15!)/(4!* (!5 - 4)!) = (15!)/(4! * 11!)`

`= (15 * 14 * 13 * 12 * (11!))/(4*3*2*1 * (11!)) = (15*14*13*12)/(4*3*2*1) = 5* 7 * 13 * 3 *1`
= 1365