We should find the number of possible ways the student can arrange 4CD from total 20 CDs. We should find the number of combinations.
Combination is a way of selecting r items (CDs) from a set (collection) of n items (CDs). The order of selection of the items does not matter in this situation.
In our case: r=4 , n=20
The formula is: C=n!/(n-r)!*r!=20!/16!*4!=20*19*18*17*16!/16!*4!=20*19*18*17/4*3*2=116280/24=4845