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$5billsinthetoprightandbottomleftsquares,asshownbelow.\begin{tabular}{|p{1em}|p{1em}|p{1em}|p{1em}|p{1em}|}
\hline
\$1 & & & & \$5 \\
\hline
& E & & &\\
\hline
& & & &\\
\hline
& & & &\\
\hline
\$5 & & & & \$1 \\
\hline \end{tabular}Everysecond,Erickrandomlychoosesasquareadjacenttotheonehecurrentlystandsin(thatis,asquaresharinganedgewiththeonehecurrentlystandsin)andmovestothatsquare.WhenErickreachesasquarewithmoneyonit,hetakesitandquits.TheexpectedvalueofErick′swinningsindollarsism/n,wheremandnarerelativelyprimepositiveintegers.Findm+n$.