# Normal distribution and standard deviation?

• Mar 6th 2009, 04:58 PM
Kalyn
Normal distribution and standard deviation?
100 studentes took a test on which the mean score was 73 with a variance of 64. a grade of A was given to all who scored 85 or better. Approximately how many A's were there, assuming scores were normally distributed?

is there anyway to solve this other than using a normal distribution table? Like, is there a formula you can use to calculate the percentage just by looking at the sd? Thank you
• Mar 7th 2009, 01:24 AM
Moo
Hello,
Quote:

Originally Posted by Kalyn
100 studentes took a test on which the mean score was 73 with a variance of 64. a grade of A was given to all who scored 85 or better. Approximately how many A's were there, assuming scores were normally distributed?

is there anyway to solve this other than using a normal distribution table? Like, is there a formula you can use to calculate the percentage just by looking at the sd? Thank you

No, I don't think there is any...

You'll have to make $Z=\frac{X-73}{\sqrt{64}}$ and then find $P(X>85)=P(Z>\tfrac{85-73}{\sqrt{64}})=\text{ look in the table}$