Can't solve this problem :(

**JollyRoger06** · Apr 14th 2010, 01:57 PM

Hey, im having trouble solving this problem, can someone explain how to do it?

The scores on an Economics examination are normally distributed with a mean of 79 and a standard deviation of 18. If the instructor assigns a grade of A to 10% of the class, what is the lowest score a student may have and still obtain an A? (Give your answer to two decimal places.)

**pickslides** · Apr 14th 2010, 02:03 PM

You need to solve for $c$ where

$P\left(Z\leq \frac{c-79}{18}\right) = 0.9$

**dkmathguy** · Apr 14th 2010, 06:44 PM

(S - μ)/σ = ?

μ = mean

σ = variance^(1/2)

? = look at the chart "percentage points of the t distribution" in your book, there is one on the back of the front cover, look at column t.100, row inf.

Solve for S.

Are you sure the standard deviation is 18?

**mr fantastic** · Apr 14th 2010, 06:50 PM

Originally Posted by dkmathguy

(S - μ)/σ = ?

μ = mean

σ = variance^(1/2)

? = look at the chart "percentage points of the t distribution" in your book, there is one on the back of the front cover, look at column t.100, row inf.

Solve for S.

Are you sure the standard deviation is 18?

This is not the correct approach to answering the question. The t-distribution has nothing to do with it. The correct approach is given in post #2.

**dkmathguy** · Apr 14th 2010, 08:15 PM

Oops, sorry about my post, I just learn how to use the Stats/List-Editor applicant on my brother graphing calculator.

Now I don't have to look up any chart from the book anymore.

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