MathDB
2010 PUMaC Combinatorics A4: expected value from grid walk

Source:

August 21, 2011
symmetryprobabilityexpected valuenumber theoryrelatively prime

Problem Statement

Erick stands in the square in the 2nd row and 2nd column of a 5 by 5 chessboard. There are \1billsinthetopleftandbottomrightsquares,andthereare$5billsinthetoprightandbottomleftsquares,asshownbelow.1 bills in the top left and bottom right squares, and there are \$5 bills in the top right and bottom left squares, as shown below. \begin{tabular}{|p{1em}|p{1em}|p{1em}|p{1em}|p{1em}|} \hline \$1 & & & & \$5 \\ \hline & E & & &\\ \hline & & & &\\ \hline & & & &\\ \hline \$5 & & & & \$1 \\ \hline \end{tabular}Everysecond,Erickrandomlychoosesasquareadjacenttotheonehecurrentlystandsin(thatis,asquaresharinganedgewiththeonehecurrentlystandsin)andmovestothatsquare.WhenErickreachesasquarewithmoneyonit,hetakesitandquits.TheexpectedvalueofErickswinningsindollarsis Every second, Erick randomly chooses a square adjacent to the one he currently stands in (that is, a square sharing an edge with the one he currently stands in) and moves to that square. When Erick reaches a square with money on it, he takes it and quits. The expected value of Erick's winnings in dollars is m/n,where, where mand and narerelativelyprimepositiveintegers.Find are relatively prime positive integers. Find m+n$.