70, 75, 80, 85, 90, 105, 105, 130, 130, 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meters. If the SD of ten running times is 22.4 seconds, rounded to the nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times?
A. one
B. two
C. three
D. four
E. five
Answer: B
Source: GMAT Prep
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meters
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Solution:Gmat_mission wrote: ↑Thu Sep 24, 2020 2:29 am70, 75, 80, 85, 90, 105, 105, 130, 130, 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meters. If the SD of ten running times is 22.4 seconds, rounded to the nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times?
A. one
B. two
C. three
D. four
E. five
Answer: B
Source: GMAT Prep
The average (mean) is:
(70 + 75 + 80 + 85 + 90 + 105 + 105 + 130 +130 +130)/10 = 1000/10 = 100
Thus, 1 standard deviation below the mean is 100  22.4 = 77.6, so there are 2 running times that are more than 1 standard deviation below the mean.
Answer: B
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