APPLY LOGIC NOT MATHEMATICS

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Amanda dies and reaches at the gate of heaven. She has three doors in front of her out of which only one of them leads to heaven. Out of the other two gates, one leads to hell for one day and then back to the gate and one leads to hell for two days and then back to the gate. If she chooses one of these two gates, the gates are shuffled when she returns back.

How long will Amanda take to reach heaven?

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    Let us simplify the question and then it will lead us to the answer easily. There are three possibilities. Thus the probability of choosing any door is 1/3rd. 1/3 of the time the door to heaven will be chosen by Amanda and thus 1/3 of the time it will take her zero days. 1/3 of the time, the day which leads to hell for one day will be chosen and of those the right door will be chosen the next day. So 1/9 trips take one day. In the similar manner, 1/9 will take to the door which takes to 2 days hell.

    When that happens, the cases will be split again and again. Thus on average, it will take three days for Amanda to reach Heaven by choosing the right gate.

    John123 Expert Answered on 19th July 2015.
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    1/3 of the time, the door to heaven will be chosen, so 1/3 of the time it will take zero days. 1/3 of the time, the 1-day door is chosen; of those, the right door will be chosen the next day, so 1/9 trips take 1 day. Similarly, 1/9 will take two days (choosing the 2-day door, then the right door).

    After that, the cases split again, and again, and again. I can’t seem to make a nice infinite sum this way, so let’s try again.

    Suppose the average days spent is X. 1/3 of the cases are done in zero days as before. 1/3 of the cases are 1 day plus X. 1/3 are 2 + X. So:

    X = 1/3 * 0 + 1/3 * (1 + X) + 1/3 * (2 + X)
    = 0 + 1/3 + X/3 + 2/3 + X/3
    = 1 + 2X/3
    Therefore,

    X/3 = 1
    X = 3
    On average, it takes three days to get to heaven. Two if the noodles are limp.

    Took me one blind alley, and about five minutes.

    Ak Scholar Answered on 5th October 2017.
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