Please help with this statistics problem

A Statistics exam is created by choosing for each question on the exam one possible version at random from a bank of possible versions of the question. There are 20 versions in the bank for each question. A specic question on the exam involves a onesample test for the population mean with hypotheses HD:,u=15 Ha:p15 with all versions of the question involving a sample of size n = 35. Seven versions of the question give the population standard deviation sis cr = 3. Six versions give the sample standard deviation as .9 = 4.2. The remaining versions give the sample standard deviation as s = 5.7. Let c'= be the critical value for the rejection region on this question. Calculate E[c']