[quote=kscheels;168572]GIven confidence level,11.24<u<12.01 and that n=90 + c= .90, find the sample standard devaiton o.
I'm assuming that since you ask for sample standard deviation rather than population standard deviation, the confidence interval has been constructed using the t-distribution:
Therefore .
In your case (I think ..... it's not really clear from what you posted), n = 90, and . Use a table of critical values to get .
Substitute all these values and solve for s.