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Math Help - Z score question

  1. #1
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    Z score question

    the time taken to for grade 4 students to complete a small puzzle follows a normal distribution with the standard deviation of 30 seconds. If 70% of grade 4 students complete the puzzle in 4 minutes or less, find the mean completion time for the grade.


    im stuck with this one i unsure how the bell shape graph would look.
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  2. #2
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    Re: Z score question

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  3. #3
    Grand Panjandrum
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    Re: Z score question

    Quote Originally Posted by realistic View Post
    the time taken to for grade 4 students to complete a small puzzle follows a normal distribution with the standard deviation of 30 seconds. If 70% of grade 4 students complete the puzzle in 4 minutes or less, find the mean completion time for the grade.


    im stuck with this one i unsure how the bell shape graph would look.
    The z-score that corresonds to 70% is ~=0.52.

    z=(4-mu)/0.5=0.52

    which you may solve for mu (in minutes).

    CB
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    Re: Z score question

    i have a chart the largest z-score i can get is 3.99
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  5. #5
    Grand Panjandrum
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    Re: Z score question

    Quote Originally Posted by realistic View Post
    i have a chart the largest z-score i can get is 3.99
    You are doing an inverse look -up in the table, you are looking for the value of z that gives an area of 0.7, or if the table is from z=0, an area of 0.2.

    CB
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