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12th Annual Harvard-MIT Mathematics Tournament #0ium

12th Annual Harvard-MIT Mathematics TournamentHard

From module Expected Value

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Understand the scale
Scale 0.5: Easiest math competition problems, often solvable even for early middle school students without experience. 1: Problems strictly for beginners, often at a moderate middle school level. 1.5: Problems for stronger beginner students, on the level of the middling problems in most middle school contests. 2: For motivated beginners, harder questions from the previous categories. 2.5: More advanced beginner problems, hardest questions from previous categories. 3: Early intermediate problems that require more creative thinking. 3.5: Tougher early intermediate problems that consistently stretch into higher-level creative thinking or conceptual knowledge. 4: Intermediate-level problems. 4.5: Upper intermediate problems approaching the upper bound for non-invitational sprint math competitions. 5: More difficult AIME problems or simple proof-based Olympiad-style problems. 6: High-level AIME-styled questions or introductory Olympiad-level questions. 7: Tougher Olympiad-level questions, may require more technical knowledge. 8: High-level Olympiad-level questions. 9: Expert Olympiad-level questions. 9.5: The hardest problems appearing on Olympiads which the strongest students could reasonably solve. 10: Historically notorious problems, generally unsuitable for even very hard competitions because they are exceedingly tedious, long, and difficult.
Expected valuesInvariants / monovariantsRecursion, bijection

Problem

Problem:

Stan has a stack of 100 blocks and starts with a score of 0, and plays a game in which he iterates the following two-step procedure:

a. Stan picks a stack of blocks and splits it into 2 smaller stacks each with a positive number of blocks, say aa and bb. (The order in which the new piles are placed does not matter.)

b. Stan adds the product of the two piles' sizes, abab, to his score.

The game ends when there are only 1-block stacks left. What is the expected value of Stan's score at the end of the game?

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