# Thread: Finding Standard deviation and mean value after finding z curve areas

1. ## Finding Standard deviation and mean value after finding z curve areas

The diameter length of contact windows used in integrated circuits is normally distributed. About 5% of all lengths exceed 3.75 micrometers and about 1% of all lengths exceed 3.85 micrometers. What is the mean value and standard deviation of the length distribution?

I did a reverse lookup of the z values and got 2 area values for x but what do I do next? I know I should be able to plug this into an equation but I am a little confused how I am suppose to do this.... any help or explanation would be greatly appreciated.

2. Originally Posted by cruxkitty
The diameter length of contact windows used in integrated circuits is normally distributed. About 5% of all lengths exceed 3.75 micrometers and about 1% of all lengths exceed 3.85 micrometers. What is the mean value and standard deviation of the length distribution?

I did a reverse lookup of the z values and got 2 area values for x but what do I do next? I know I should be able to plug this into an equation but I am a little confused how I am suppose to do this.... any help or explanation would be greatly appreciated.
remember, $\displaystyle z = \frac {X - \mu}{\sigma}$

Plug in your z values and the corresponding X values. You will then have two equations with two unknowns, $\displaystyle \mu \text{ and } \sigma$. Solve the system simultaneously to find their values.