# distribution

• Jan 9th 2007, 09:25 AM
jimmybob
distribution
Thank for all the help on the previous subject, now got one on distribution..

In an examination it is known that the distribution of marks is N (52,24)

a) what proportion of the marks will exceed 55?
b) what proportion of the marks will be less than 45?

c) if a grade A is awarded for the to p5% of marks, what is the minimum mark required in order to achieve a grade A?
d) the bottom 20% of the marks was graded as an F, What was the range of these marks?

Sorry for all the questions, had hundreds of these and stuck on a few of them :(
• Jan 12th 2007, 02:22 PM
F.A.P
Quote:

Originally Posted by jimmybob
Thank for all the help on the previous subject, now got one on distribution..

In an examination it is known that the distribution of marks is N (52,24)

a) what proportion of the marks will exceed 55?
b) what proportion of the marks will be less than 45?

c) if a grade A is awarded for the to p5% of marks, what is the minimum mark required in order to achieve a grade A?
d) the bottom 20% of the marks was graded as an F, What was the range of these marks?

Sorry for all the questions, had hundreds of these and stuck on a few of them :(

You probably have a table of values for the standard normal distribution N(0,1). Now if X = N(52,24) you can normalize

$\displaystyle P(X<x)=P\left(\frac{X-52}{\sqrt{24}}<\frac{x-52}{\sqrt{24}}\right)=P\left(Z<\frac{x-52}{\sqrt{24}}\right)$

where Z is standard normal N(0,1). Now you look for the corresponding value in table...

Using this you should be able to solve a-d.