Suppose we wish to test the hypotheses
H0 :  15
H1 :   15
where we know that 2 = 9.0. If the true mean is really 20, what sample size must be used to ensure that the probability of type II error is no greater than 0.10? Assume that  = 0.05.
What n is needed such that the Type II error, , is less than or equal to 0.10?