第3题

Twenty-one girls 且twenty-one boys took part 中a mathematical contest. Each contestant solved 在most six problems. F或每个girl 且每个boy, 在least one problem was solved 由both 的them. 证明 there was a problem solved 由在least three girls 且在least three boys.

第 2/5 步：Label grid

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通俗地说

译文：Every girl–boy pair has a shared problem.

21^2=441

详细分析

译文：Make a $21\times21$ grid. In 每个cell choose one problem solved 由corresponding girl 且boy.