# Can't solve this problem :(

• April 14th 2010, 02:57 PM
JollyRoger06
Can't solve this problem :(
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.)
• April 14th 2010, 03:03 PM
pickslides
You need to solve for $c$ where

$P\left(Z\leq \frac{c-79}{18}\right) = 0.9$
• April 14th 2010, 07:44 PM
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?
• April 14th 2010, 07:50 PM
mr fantastic
Quote:

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.
• April 14th 2010, 09:15 PM
dkmathguy
Oops, sorry about my post, I just learn how to use the Stats/List-Editor applicant on my brother graphing calculator.(Giggle) Now I don't have to look up any chart from the book anymore.(Rofl)