8. An electrical firm manufactures light bulbs that have a length life with normal distribution, and a standard devia- tion of 40 hours. A sample of size 15 is obtained, and its sample mean is calculated to be 780 hours. (a) Find an upper 95% bound for the average length life. (b) Find an lower 95% bound for the average length life. 9. In a psychological testing experiment, 25 subjects are selected randomly an their reaction time in seconds to a