4. A life insurance salesman believes that the mean age of people who buy their first life insurance plan is less than 35. To test his belief he takes a random sample of 15 customers who have just purchased their first life insurance. Their ages are shown, and assume to be normally distributed: 42, 43, 28, 34, 30, 36, 25, 29, 32, 33, 27, 30, 22, 37, and 40. Can we conclude at the 1% significance level that the insurance salesman is correct? (6 marks)