Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2022/discrete-mathematics-continuous-mathematics-and-probability/discrete-mathematics/2/a/iii/solution

By the generalized pigeonhole principle, 13 cards guarantee cards in one suit. The guarantee is sharp: 12 cards can consist of three cards from each suit, with no four cards of a common suit.

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