A slip of paper is given to a person $A$ who marks it either with a plus sign or a minus sign. The probability of his writing a plus sign is $1/3$. A passes the slip to $B$, who may either leave it alone or change the sign before passing it to $C$. Next $C$ passes the slip to $D$ after perhaps changing the sign. Finally $D$ passes it to a refere after perhaps changing the sign. $B,C,Deachchan\ge thesignwithprobability$2//3$.Theprobabilityt\hat{t}herefereeobservesaplussignontheslip\text{if}itisknownt\hat{}$A\displaystyle{w}{r}{o}{t}{e}{a}{p}{l}{u}{s}{s}{i}{g}{n}{i}{s}

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