Tantalizer 428: Sisters of mercy
From New Scientist #979, 11th December 1975 [link]
Faith, Hope and Charity had “adopted” an old couple in their neighbourhood and a random one of the drops in each morning to jolly things along. Tom and Annie, the oldsters, take it in good part, especially since they started having a flutter on who the next ministering angel will be.
“Tell you what”, Tom proposed slyly one evening, “Let’s have an extra bet. Who do you bet it will be for the next two days?”
“Faith both days”, said Annie.
Tom replied, “And I bet it will be Hope, followed by Faith. £1?”
“Very well”, said Annie, “but what if we are both wrong?”
“Then the bet stands until such time as Faith arrives either for the second day running (and you win) or on the day after Hope (and I win).”
“Done”, said Annie.
What are Tom’s chances of winning?