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Finding a prize door with two hint spins

Question

One of 33 doors hides a prize of $1\text{\char36}1 million; the others are empty. Before choosing, you spin a wheel twice. On each spin, independently, the wheel reveals the prize door with probability 35\tfrac35 and otherwise reveals nothing. If neither spin reveals it, you pick one of the three doors uniformly at random. What is the probability that you end up with the prize?

Short answer

6775\tfrac{67}{75}. Both spins are uninformative with probability (25)2=425(\tfrac25)^2=\tfrac4{25}; otherwise you know the door. So the success probability is 2125+425⋅13=6775\tfrac{21}{25}+\tfrac4{25}\cdot\tfrac13=\tfrac{67}{75}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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