Testing for Population mean.
The average fee charged by a large accounting firm to prepare an individual income tax return is $250 and the standard deviation (σ) is $12. Joan Smith is a CPA who works for the firm and her supervisor thinks she may be charging her clients significantly more for their tax returns. On a random sample of 40 tax returns that Joan completed last year, the average cost was $270. Based on this sample evidence, is Joan's supervisor correct? Use a = .05.