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Harvard-MIT Mathematics Tournament #0itn

Harvard-MIT Mathematics TournamentHard

From module Generating Functions (Intro)

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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.
Generating functions

Problem

Problem:

On an infinite chessboard (whose squares are labeled by (x,y)(x, y), where xx and yy range over all integers), a king is placed at (0,0)(0,0). On each turn, it has probability of 0.10.1 of moving to each of the four edge-neighboring squares, and a probability of 0.050.05 of moving to each of the four diagonally-neighboring squares, and a probability of 0.40.4 of not moving. After 20082008 turns, determine the probability that the king is on a square with both coordinates even. An exact answer is required.

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