Question 4 (25 marks)
A candy factory produces a popular candy. Suppose that the weight of a bag of candy from the production line of the factory follows a normal distribution with mean 371 grams and standard deviation 23 grams.
(a) What is the probability that a bag of candy weighs between 365 and 380 grams?
(b) As a routine check, a random sample of 55 bags of candy is selected. What is the probability that the sample mean weight is greater than 380 grams?
(c) Suppose the selected 55 bags of candy in (b) gave a mean 373.2 grams and a standard deviation 18.63 grams. At 1% level of significance, test whether the mean weight is greater than 371 grams.
(d) The production engineer wants to maintain the mean at 371 grams and adjust only the value of standard deviation so that only 4% of the weight of a bag is greater than 400 grams. Find the adjusted standard deviation.