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Math Help - made a mistake somewhere

  1. #1
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    made a mistake somewhere

    cars arrive according to a poisson distribution with a mean of 80 cars per hr. how long can the car attendant make a phone call if the probability is at least 0.4 that no cars arrive during the call?

    my working:

    let x be the time that the car attendant can make the call.

    then X ~ P(80/60 x)

    then P(X = 0) > 0.4 indicates from the statistic table that the mean has to be less than 0.90 and hence

    80/60 x = 0.90 means that x = 40 sec

    but the answer in my book states that the time is 23 sec.

    is there something wrong with my reasoning?
    Last edited by alexandrabel90; March 6th 2010 at 12:09 PM.
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