A large number of light bulbs were turned on continuously to determine the average number of days a bulb can last. The study revealed that the average lifetime of a bulb is 120 days with a standard deviation of 10 days. If the lifetimes are assumed to be independent normal random variables, find the confidence limits for a confidence level of 90% on the sample mean that is computed from a sample size of (a) 100 and (b) 25.