Question

Neo Anderson wants to purchase six different CDs but only has enough money to purchase four. In how many ways can he select four of six CDs for purchase?

Answer 1

Assume that A B C D E F are the 6 CDs
Is there any difference between selecting A B C D and A C B D?

Answer 2

Luis's right! Without repetition so just 6!/ 2!

Answer 3

I think the key point is the importance of order
Both nCr and nPr can be used for the case 'without repetition'
For nPr, the order is important. In Luis's example, the question has already stated that 'different ordering options', that mean order is important in that case, for instance, A B C D is NOT equal to A C B D.
but for this case, I think buying the CDs in the order A B C D is the same as A C B D, that means order is not important. So, personally, I think nCr is more appropriate to use in the calculation.