Posted by Kassie on .
Suppose X is a normally distributed random variable with the mean u(Mu) and variance sigma^2. Suppose P(X </= 10)  .6915 and P (X </= 20) .8413. Find Mu and Sigma^2.
PLEASE HELP! THIS IS AN EXAM REVIEW QUESTION!

Statistics 
Reiny,
for zscores,
z = (x  m)/s
from my charts, a prob of .6915 corresponds to a zscore of .5
so .5 = (10  m)/s
.5s = 10m
m = 10  .5s
and a prob of .8413 corresponds to a zscore of .9998 or 1
1 = (20m)/s
s= 20m
m = 20  s
then 20  s = 10  .5s
s = 10/.5 = 20
m = 0
If I recall variance is the square of the standard deviation.
I checked my answers with
http://davidmlane.com/hyperstat/z_table.html