I stuck when finding the critical point. here it is:
A new brand of canned mixed nuts plans to enter the Singapore market. The label says that 25% of the contents are cashews. Suspecting that this might be an overstatement, an inspector takes a random sample of 31 cans and measures the percentage weight of cashews [i.e., 100(weight of cahews)/(weight of all nuts)] in each can. The mean and standard deviation of these measurements are found to be 23.5 and 3.1, respectively. Do these results constitute strong evidence in support of the inspector’s belief? Test with = 0.01.