A researcher wished to evaluate the operating life of a certain brand of storage battery. A group of 30, brand-new batteries was selected and placed in 30 identical automobiles. The lights in each automobile were then turned on (without running the engines), and the length of time before the batteries were fully discharged (lights went out) was recorded. The scores, in hours, were: 15, 10, 20, 18, 18, 11, 11, 11, 11, 12, 12, 12, 10, 9, 9, 8, 9, 9, 11, 11, 11, 15, 12, 12, 12, 10, 12, 12, 15, 12. What is the mean