# Math Help - Find the Mean

1. ## Find the Mean

If X is a normal variable with a standard deviation of 2 and Pr(X<9) = 0.8
Find the mean.

Im new to the whole normal distribution thing so i would really appreciate some help.

thanks

2. Originally Posted by Kash
If X is a normal variable with a standard deviation of 2 and Pr(X<9) = 0.8
Find the mean.

Im new to the whole normal distribution thing so i would really appreciate some help.

thanks
Find the value z* of Z such that Pr(Z < z*) = 0.8.

Substitute this value into $Z = \frac{X - \mu}{\sigma}$:

$z* = \frac{9 - \mu}{2}$.

Solve for $\mu$.

3. wow i didnt know that formula!, thanks alot!