probabilities of normal distribution as a function of unknown standard deviation.
I have the following problem.
X follows a normal distribution with mean 10 and unknown standard deviation. I would like to find the standard deviation, for which the probability P(8<X<11) is 0.7.
I tried to graph this probability as a function of the standard deviation, but I got an error message.
Am I doing something wrong? Is it possible to put the unknown in the position of the standard deviation? If not, how can this be done without actually using the formula of the normal curve?