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  1. #1
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    help

    Historically, final exam scores in regular session Psychology 281 have been normally distributed with a mean of 72%. 95% of all students obtain scores between 52 and 92.

    What percentage of the students obtain scores between 80 and 86? [13.24%]

    If a random sample of 45 Psych 281 students was selected at random, what is the probability that the sample's mean final exam score would be greater than 68? [0.9957]
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  2. #2
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    Quote Originally Posted by skhan
    Historically, final exam scores in regular session Psychology 281 have been normally distributed with a mean of 72%. 95% of all students obtain scores between 52 and 92.

    What percentage of the students obtain scores between 80 and 86? [13.24%]

    If a random sample of 45 Psych 281 students was selected at random, what is the probability that the sample's mean final exam score would be greater than 68? [0.9957]
    You need to find standard deviation first. Since 95% is two standard deviations you have,
    72+2\sigma=92 thus, \sigma=10.
    To find P(80\leq x\leq 86) find,
    P(72\leq x\leq 86)-P(72\leq x \leq 80)
    You do this by finding the z-scores which are,
    z=1.4,.8 respectively. Looking up at the charts we have, that 1.4 gives .4192 and .8 gives .2881 thus, subtract them to get, 13.11%
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  3. #3
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    Quote Originally Posted by ThePerfectHacker
    You need to find standard deviation first. Since 95% is two standard deviations
    In fact 95% corresponds to about +/-1.96 standard deviations.

    RonL
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  4. #4
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    Quote Originally Posted by CaptainBlack
    In fact 95% corresponds to about +/-1.96 standard deviations.

    RonL
    Explains why there is a small discrepency between my answers and his book's answer.
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