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Probability problem - finding standard deviation and mean in a normal dist.

  1. Oct 27, 2006 #1
    Hey all.

    Im doing maths methods 5 (i live in Australia) and I've run into this problem.
    Code (Text):

    A university study investigated the increase in
    heart rates(measured in beats per minute) of
    people undertaking a particular exercise. The
    increases in heart rate were normally distributed
    with a mean of 40 and a standard deviation of 9.

    It was determined that the proportion of people
    classified as unfit (x >= 50) is 15% and the
    proportion of people classified as very fit (x <= 22) is 10%.
    Find the mean and standard deviation of the increases
    in heart rate for this university study.
    I've no idea how to do this.

    Most of these types of problems I have been doing on a graphics calculator, which is allowed and expected in this course. The model of calculator I have is a CASIO fx-9860G AU.

    Any help in solving this problem will be greatly appreciated.

    Thanks for reading.
  2. jcsd
  3. Oct 28, 2006 #2
    Its okay, I got it figured out.

    I took the z scores for those percentages as if they were standard normal distributions and then substituted them into the formula z = (x - mu)/sigma. I then had simultaneous equations which were quite easy to solve. I thought i'd answer my own question for anyone who wanted future reference.
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