4. A research group studied personal consumption (in dollar) for customers in a mar- ketplace. They randomly pick 9 customers and ask for their cost X, X2. ..., X,, the sample average X = 131. Assume the individual consumption follows A(/, 9).(c) Now we can further increase the sample size, to make sure 8 $ 0.05, at least how large the sample size should be? (Hint: for common standard normal cutoff, please refer to Lecture notes 26.)