A measure of tire wear is the percentage of tire groove lost by the tire since new. A senior JJ research student tried to estimate the average percentage groove lost for JJ5 tires after 30000 miles. She randomly selected 15 tires with 30000 miles of wear on each tire. These are the percentage of tire groove lost on each: 35.7, 39.2, 36.1, 28.1, 24.0, 28.7, 25.9, 11.5, 15.2, 19.9, 16.1, 20.9, 23.7, 23.1, 23.3 Percentage groove lost after 30000 miles is known to be normally distributed with an unknown mean of u and an unknown standard deviation a a) Calculate the sample mean for this data.

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