The mean shoe size of the students in a math class is 7.5. Most of the shoe sizes fall within 1 standard deviation, or between a size 6 and a size 9.

One standard deviation below the mean is size 6, and one standard deviation above it is size 9. Each endpoint is 1.5 sizes from the mean of 7.5 (7.5 − 6 = 1.5 and 9 − 7.5 = 1.5). Therefore, the standard deviation is 1.5, choice A.

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