Sarah is a video game designer currently working on a fighting game. She'd like to have an animated and voiced "face-off" between the fighters before each match starts. This interaction will be unique for every possible 1v1 match-up among the game's 10 characters. How many of these interactions will she have to design?
Note : In this fighting game, it is possible for a character to be matched up against the same character. The order of the fighters does not matter (if the order is switched, the animation is simply mirrored, and this does not count as an additional animation).
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Easily 2 cases:
If they choose the same character, there are only 1 0 pairs, one for each character.
If they choose different characters, then the interaction is the same for each pair chosen, without order. As follows:
( 2 1 0 ) = 2 ! ⋅ 8 ! 1 0 ! = 2 ⋅ 8 ! 1 0 ⋅ 9 ⋅ 8 ! = 5 ⋅ 9 = 4 5
Therefore, the total amount of pairs are 1 0 + 4 5 = 5 5