final answer please

Use the following info for this question: A survey of 500 randomly selected adult men showed that the mean time they spent per week watching sports on television is 9.75 hours with a standard deviation Of 2.2 hours. Construct a 80% confidence interval for the population mean u, i.e. (a, b). How large a sample size of necessary if the length of 80% interval is to be 0.4? [answer should be in integer]