2. 外语
  3. Closing prices for a certain stock were recorded each day for a week in the table above. If the monthly average (arithmetic mean

Closing prices for a certain stock were recorded each day for a week in the table above. If the monthly average (arithmetic mean

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问题 Closing prices for a certain stock were recorded each day for a week in the table above. If the monthly average (arithmetic mean) was 18.43 and the standard deviation was 0.27, how many days during this week did the closing price fall within two standard deviations from the average?
选项 A、1
B、2
C、3
D、4
E、5
答案C
解析 Since two standard deviations away from the mean works out to 18.43-0.27×2=17.89, there are 3 days during which the stock prices fell between 17.89 and 18.43: Monday, Tuesday, and Wednesday. We really don’t have to worry about prices above the mean, because there were none of them during this time frame.
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