If 68% of the observations of a bell-shaped, symmetrical frequency distribution fall between -20 and + 60, the mean and the standard deviation of the distribution are:
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A. B. C. D.D
Since the distribution is symmetrical about the mean, the mean is equal to (-20 + 60)/2 = 20. According to the empirical/Normal rule, for a bell-shaped, symmetrical frequency distribution, 68% of the observations lie within one standard deviation of the mean. Therefore, the standard deviation equals 60-20 = 40.