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Normally distributed random variable and probability

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data
    The top-selling Red and Voss tire is rated 60000 miles, which means nothing. In fact, the distance the tires can run until wear-out is a normally distributed random variable with a mean of 70000 miles and a standard deviation of 5000 miles.

    A: What is the probability that the tire wears out before 60000 miles?
    B: What is the probability that a tire lasts more than 79000 miles?

    2. Relevant equations
    z = (Y-μ)/σ

    3. The attempt at a solution
    I plugged the values into the above equation and got z=-2. Looking at the chart in my book, Pr(z=2)=0.0228 and I figured this should work since z is 2 standard deviations from the mean no matter whether it's positive or negative. When I submitted 0.0228 though, the answer was incorrect.
     
  2. jcsd
  3. Oct 24, 2011 #2

    Ray Vickson

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    Science Advisor
    Homework Helper

    Your answer is correct. Who told you it is incorrect?

    RGV
     
  4. Oct 25, 2011 #3
    It's an online homework assignment. I talked to my teacher and he said the site was having issues with the rounding. Thanks for the help!
     
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