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Homework Help: A problem on statistics

  1. Oct 11, 2011 #1
    1. The problem statement, all variables and given/known data

    An airline flies the same route at the same time each day. The flight time varies according to a Normal distribution with unknown mean and standard deviation. On 15% of days, the flight takes more than an hour. On 3% of days, the flight lasts 75 minutes or more. Use this information to determine the mean and standard deviation of the flight time distribution.

    2. Relevant equations

    well, i can use the normalcdf, normalpdf and invNorm on the TI, but i'm not sure how, since to use them, i need the standard deviation and the mean

    3. The attempt at a solution

    what i have attempted is using those tools on the calculator, but it doesn't really get you to anything
  2. jcsd
  3. Oct 11, 2011 #2


    Staff: Mentor

    Some tools you should be using before you start in with the calculator are a pencil and some paper, and a table of values of the standard normal distribution.

    You are given that the flight times are normally distributed, with mean [itex]\mu[/itex] and standard deviation [itex]\sigma[/itex], both unknown.

    Let X be the normally distributed random variable that represents flight times.

    You are given that Pr(X > 60) = .15 and Pr(X > 75) = .03

    How can you write these probabilities so that they involve Z, the standard normal random variable? What's the relationship between Z and X?
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