4. The life span of an electronic chip used in a high-powered laptop is estimated to be 625.35 hours from a random sample of 40 chips. The life of an electronic chip is considered to be normally distributed with a population variance of 400 hours. a. Find a 90% confidence interval for the mean life of the electronic chips. (8 pts) b. Is the true mean life of the electronic chips different from 633 hours? Use a 10% significance level. (15 pts)