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MathNet 2019-part-08 — 179 problems (EN–VI) Question 118

Math Olympiad Vault · MathNet 2019-part-08 — 179 problems (EN–VI)

Question 118

James is standing at the point $(0,1)$ on the coordinate plane and wants to eat a hamburger. For each integer $n \geq 0$, the point $(n, 0)$ has a hamburger with $n$ patties. There is also a wall at $y=2.1$ which James cannot cross. In each move, James can go either up, right, or down 1 unit as long as he does not cross the wall or visit a point he has already visited.

Every second, James chooses a valid move uniformly at random, until he reaches a point with a hamburger. Then he eats the hamburger and stops moving. Find the expected number of patties that James eats on his burger.

Proposed by: Joey Heerens