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Math Help - Could someone check my review question? Probability

  1. #1
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    Could someone check my review question? Probability

    There is going to be a question like this on a test coming up and I just wanted to see if I was doing it right. Could somebody maybe check my answer for me? Thanks
    p.s. If I did it wrong can you show me how to do it?



    The time it takes to travel from home to the office is normally distributed with mean = 25 minutes and SD = 5 minutes.

    A)What is the probability the trip takes more than 20 minutes?

    my answer---X=20, z=(20-25)/5=-1
    Z score of -1 is .3413
    .5-.3413=.1587
    The probability that the trip takes more than 20 minutes is .1587 or 15.87%.


    B)What is the probability the trip takes less than 15 minutes?

    my answer-X=15, z=(15-25)/5=-2
    Z score is .4772
    5-.4772=.0228
    The probability that the trip takes less than 15 minutes is .0228 or 2.28%.




    C)What is the probability the trip takes between 30 and 35 minutes?

    my answer-x=30, z=(30-25)/5=1
    x=35, z=(35-25)/5=2
    p(30<x<35)=P(1<z<2)

    P(30<x<35)=(.3413<x<.4772)
    .4772-.3413=.1359 or 13.59%
    The probability that the trip takes between 30 and 35 minutes is .1359
    Or 13.59%.



    D)What is the probability the trip takes more than 40 minutes?

    my answer-X=40, z=(40-25)/5=3
    Z score of 3 is .4986
    .5-.4986=.0014
    The probability that the trip will take more than 40 minutes is .0014 or .14%.
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  2. #2
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    Quote Originally Posted by jwells1999 View Post
    There is going to be a question like this on a test coming up and I just wanted to see if I was doing it right. Could somebody maybe check my answer for me? Thanks
    p.s. If I did it wrong can you show me how to do it?



    The time it takes to travel from home to the office is normally distributed with mean = 25 minutes and SD = 5 minutes.

    A)What is the probability the trip takes more than 20 minutes?

    my answer---X=20, z=(20-25)/5=-1
    Z score of -1 is .3413

    Mr F says: More than 20 minutes .....You want Pr(X > 20), which boils down to Pr(z > -1).

    .5-.3413=.1587 Mr F says: This is Pr(z < -1). You want Pr(z > -1) ......

    The probability that the trip takes more than 20 minutes is .1587 or 15.87%.

    Mr F says: You should have ralised immediately that this answer is wrong. 20 is less than the mean. So how on Earth can Pr(X > 20) be LESS than 0.5??



    B)What is the probability the trip takes less than 15 minutes?

    my answer-X=15, z=(15-25)/5=-2
    Z score is .4772
    5-.4772=.0228
    The probability that the trip takes less than 15 minutes is .0228 or 2.28%. Mr F says:



    C)What is the probability the trip takes between 30 and 35 minutes?

    my answer-x=30, z=(30-25)/5=1
    x=35, z=(35-25)/5=2
    p(30<x<35)=P(1<z<2)

    P(30<x<35)=(.3413<x<.4772)
    .4772-.3413=.1359 or 13.59%
    The probability that the trip takes between 30 and 35 minutes is .1359
    Or 13.59%. Mr F says: Although the setting out of your working leaves a bit to be desired ......



    D)What is the probability the trip takes more than 40 minutes?

    my answer-X=40, z=(40-25)/5=3
    Z score of 3 is .4986
    .5-.4986=.0014
    The probability that the trip will take more than 40 minutes is .0014 or .14%. Mr F says: I get 0.00134996722, which will round to 0.0013, correct to 4 decimal places. Or 0.13% .......
    ..
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  3. #3
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    Does this look right?

    X=20, z=(20-25)/5=-1
    Z score of -1 is .3413
    .3413+.5=.8413
    The probability that the trip takes more than 20 minutes is .8413 or 84.13%

    Does this look right for the first part. This is really trippin me up. I dont know why.
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  4. #4
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    Quote Originally Posted by jwells1999 View Post
    X=20, z=(20-25)/5=-1
    Z score of -1 is .3413
    .3413+.5=.8413
    The probability that the trip takes more than 20 minutes is .8413 or 84.13% Mr F says:

    Does this look right for the first part. This is really trippin me up. I dont know why.
    I find drawing simple pictures and shading in the area(s) of the things I want helps.
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