[問題]GMATPrep PP 關於標準差

[achiachia]

70, 75, 80, 85,90, 105, 105, 130, 130, 130

The list shown consists of the time, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters. If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

A)one
B)two
C)three
D)four
E)five

請問，題目在問什麼？謝謝

[medi66]

請問 答案是???

[StanleyHuang]

我猜最後的問句少一個字，應該是how many of the 10 running times that are more than 1 standard deviation below the mean of the 10 running times?
意思就是說，超過一個標準差的而且小於平均的有幾個？
==>先出算平均 = 100
==>一個標準差的範圍 = (100-22.4) <-> (100+22.4) ==>77.6 <-> 122.4
所以所求就是 70, 75 這兩個。選(B)

[achiachia]

太感謝了！