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MathNet 2024-part-01 — 168 problems (EN–VI) Question 121

Math Olympiad Vault · MathNet 2024-part-01 — 168 problems (EN–VI)

Question 121

There are 8 cards on the table numbered from 1 to 8. Two players, $A$ and $B$, play the following game. In each round:

* Player $A$ selects two cards from the table
* Player $B$, after seeing the two selected cards, chooses one to keep and discards the other

The game consists of four rounds with the restriction that:

* In rounds 1 and 2, $B$ cannot choose the larger number in both rounds
* In rounds 3 and 4, $B$ cannot choose the larger number in both rounds

Let $S$ be the sum of the numbers on the four cards $B$ holds after four rounds.

Find the largest integer $N$ such that no matter how $A$ selects cards in each round, $B$ can guarantee $S \ge N$.