Statistics is not my strong point. How do you do this one:- The marks obtained by the class in a test are normally distributed with mean 74 and standard deviation 15. The top 10% get an A grade. Calculate the minimum mark required to get A grade.

Question

celticcat · Answer

I guess you need to use the standardised score equation  z  = (X - 74)  / 15  but how do you get the z score?

Zowie · Answer

z would be 0.1 because it's 10% but you have to change it to a decimal. When changing a percent to a decimal you move the . over 2 places to the right.

celticcat · Answer

But that gives an answer of  X = 75.5  but the correct answer is 94.

celticcat · Answer

we have to use the Normal  Distribution tables I guess...

celticcat · Answer

Maybe I should be looking at the area under the  Normal Cheeseulative  curve  of > 0.9???

celticcat · Answer

What???? The computer isnt printing what Im typing!!!

Zowie · Answer

It is. I just have no clue what you are talking about. Wouldn't want to give you a bad/wrong answer.

celticcat · Answer

Yes thats fine.  I meant to type  C u m u l a t i ve.

Zarkon · Answer

Solve $\displaystyle\Phi^{-1}(.90)=\frac{x-74}{15}$ for $x$

where $\displaystyle\Phi^{-1}(\cdot)$ is the inverse standard normal CDF.

celticcat · Answer

Ah right   Than you.