Normal distribution and standard deviation

godales77

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Mar 2, 2010
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Can someone please help me with this problem. It's been killing me all day. I do not know how to find the first or third quartiles. Thanks. :/

The scores of students on an exam are normally distributed with a mean of 219 and a standard deviation of 69.

(a) What is the first quartile score for this exam?
Answer:

(b) What is the third quartile score for this exam?
Answer:
 

tutor_joel

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Feb 6, 2010
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So, the first quartile means that you want to know what the 25th percentile is for the given mean and the third quartile is the 75th percentile. Using your z table, look up the z value for the 25th percentile for part a. That is, the value 0.25 corresponds to what z value? It should be about -0.675. Do you know how to find this? Then just use this equation:

\(\displaystyle z=\frac{x - \mu}{\sigma}\)

You're solving for x here given mu and sigma and looking up z in the table.

\(\displaystyle x = z\sigma + \mu\)

In order to do part b, look up the z value for the 75th percentile and re-solve.

You should know that the first quartile should be a very low score and the 3rd quartile should be much larger. Just to get a sense if your answer is correct.
 

kpsingh

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Jan 22, 2017
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The scores of students on an exam are normally distributed with a mean of 219 and a standard deviation of 69.

(a) What is the first quartile score for this exam?
Answer:

(b) What is the third quartile score for this exam?
Answer:[/QUOTE]
 
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