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その5人を仮に、AさんBさんCさんDさんEさんとすると
AとB、AとC、AとD、AとE、BとC、BとD、
BとE、CとD、CとE、DとE
ペアが10個出来る
A.10通り
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11じゃなくて10だと思うよ