问题
单项选择题
An analysis determined that approximately 99 percent of the observations of daily sales for a company were within the interval from $ 250000 to $ 580000 and that daily sales for the company were normally distributed. The mean daily sales and standard deviation of daily sales, respectively, for the company were closest to: Mean daily sales Standard deviation of daily sales（）
A. $ 415000 $ 41667
B. $ 415000 $ 55000
C. $ 355115 $ 41667
答案
参考答案：B
解析：
Given that sales are normally distributed, the mean is centered in the interval mean=($ 250000+ $ 580000)/2 =$ 415000. An interval including 99% of the observations extends three standard deviations either side of the mean. The standard deviation of daily sales=($ 415000-$ 250000)/3=$ 55000.