8. A phone maker claims that its phones' batteries last for 13.8 hours on average with standard usage with a standard deviation of 1.2 hours. The distribution of battery lives is normal (a) If we accept that claim, what is the probability that a battery charge lasts between 15 and 17 hours? (b) Find the 85, percentile for the battery lives- that is, a time for which 85% of the batteries last less than that time. (c) The company has just tested 19 devices. Find the probability that the mean battery life of the 19 phones is less than 13 hours