30% of taking out both reds. (40% you have one in front of you at that point.)
You're left with two boxes from five. The probability that you've got a blue is 60%. If you have a blue in front of you, then it's a 50% chance another blue is left to the end. Hence the all-blue finish is a 30% chance (60% * 50%).
You're right that the offer was awfully generous, yet at the same time it was easy to decline at the relatively low stakes and with the spanking still on from going to the end. People usually get carried away on this sort of board, even if they're not as vocally aggressive as John was on the wings.
(For what it's worth, I invented the "fair deal" metric, back in 2005. Think it was first used in Anita's game which aired on Challenge again on Tuesday.
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