A23 - In class sample size problem: A company has set a goal of developing a battery that lasts five hours (300min) in continuous use. In a first test, eight of these batteries measured the following lifespans (in min.): 321, 295, 332, 351, 281, 336, 311, 253. How many batteries must we test to be 99% sure of estimating the mean lifespan to within 15 minutes