SAT Writing scores are normally distributed with a mean of 491 and a standard deviation of 113.A university plans to send letters of recognition to students whose scores are in the top 8%. What is the minimum score required for a letter of recognition?

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

The Z-score is "a numerical measurement used in statistics of a value's relationship to the mean (average) of a group of values, measured in terms of standard deviations from the mean".

Solution to the problem

Let X the random variable that represent the scores of a population, and for this case we know the distribution for X is given by:

[tex]X \sim N(491,113)[/tex]

Where [tex]\mu=491[/tex] and [tex]\sigma=113[/tex]

For this part we want to find a value a, such that we satisfy this condition:

[tex]P(X>a)=0.08[/tex] (a)

[tex]P(X<a)=0.92[/tex] (b)

Both conditions are equivalent on this case. We can use the z score again in order to find the value a.

As we can see on the figure attached the z value that satisfy the condition with 0.92 of the area on the left and 0.08 of the area on the right it's z=1.405. On this case P(Z<1.405)=0.92 and P(z>0.92)=0.08

If we use condition (b) from previous we have this:

[tex]P(X<a)=P(\frac{X-\mu}{\sigma}<\frac{a-\mu}{\sigma})=0.92[/tex]

[tex]P(z<\frac{a-\mu}{\sigma})=0.92[/tex]

But we know which value of z satisfy the previous equation so then we can do this:

[tex]z=1.405<\frac{a-491}{113}[/tex]

And if we solve for a we got

[tex]a=491 +1.405*113=649.765[/tex]

So the value of height that separates the bottom 92% of data from the top 8% is 649.765.