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Eight football teams participate in a tournament of one round (each team plays each other team once) . There are no draws. Prove that it is possible at the conclusion of the tournament to be able to find

4

teams , say

A, B, C

and

D

so that

A

defeated

B, C

and

D, B

defeated

C

and

D

, and

C

defeated

D

(097) 1

Problems(1)