The answer to the question is 105, not 2520. I posted a solution on another forum, which I'll paste here:
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Think of this question:
A group of eight tennis players will be divided into four teams of two. One team will play in the Olympics, one in Wimbledon, one in the Davis Cup and one in the US Open. In how many different ways can the teams be selected? Here, the order of the teams themselves clearly matters. If we choose {A,B} to go to the Olympics, and {C,D} to go to Wimbledon, that's clearly different from sending {C,D} to the Olympics and {A,B} to Wimbledon. The answer to this question is exactly the answer you give above:
-you have 8C2 choices for the Olympics team;
-you have 6C2 choices for the Wimbledon team;
-you have 4C2 choices for the Davis Cup team;
-you have 2C2 (one) choice for the US Open team.
Multiply these to get the answer: 8C2*6C2*4C2*2C2 = (8*7/2)(6*5/2)(4*3/2)(2*1/2) = 2520.
Note that the question I've just asked above is different from the question in the original post. In this question:
A group of 8 friends want to play doubles tennis. How many different ways can the group be divided into 4 teams of 2 people? the order of the teams does
not matter. If we choose, say, these teams:
{A,B}, {C,D}, {E,F}, {G,H}
that's exactly the same set of teams as these:
{C,D}, {A,B}, {G,H}, {E,F}
Because the order of the teams themselves does not matter, we must divide by 4! = 24, the number of different orders we can put the four teams in, because all 24 different orders are in fact the same set of teams. So the answer is 2520/4! = 105.
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75% (01:34) correct
