Combination problem with repetition. GRE Anagram Grid questions. Why do we divide?

Question

If 3 of 7 standby passengers are selected for a flight, how many different combinations of standby passengers can be selected? So the answer to do this I think is 7! / (4! * 3!) The standby passengers **and** the flying passengers are both indistinguishable or rather order doesn't matter. But again, why aren't we subtracting? Why are we dividing?

Answer 1

The items can be arranged 6 ways:

ABC

ACB

BAC

BCA

CAB

CBA

Now the order of the passengers does not make a difference. So the combination is only 1/6 of the permutation..