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You come home after dark, and porch light is out. You cannot tell which key opens the door.. you randomly try the 5 keys on your ring, 2 keys open your door.

Draw a tree diagram for the experiment &
Find the probability of opening the door on the first or second try.

  • Math -

    prob of picking the right key = 2/5

    prob of opening on first try = 2/5
    prob of opening on 2nd try = (3/5)(2/5) = 6/25
    (2nd try has to be Miss, Success)

    prob of 1st OR 2nd try
    = 2/5 + 6/25 = 16/25

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