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by rnaah » Tue Oct 11, 2011 4:01 am
The list below shows the times taken by 10 students to run a 400 meters race. if the standard deviation is 22.4, how many of the times are above one standard deviations below the mean.
70, 75, 80, 85, 90, 105, 105, 130, 130, 130
a) two
b) three
c) four
d) five
e) one
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by Geva@EconomistGMAT » Tue Oct 11, 2011 5:19 am
rnaah wrote:The list below shows the times taken by 10 students to run a 400 meters race. if the standard deviation is 22.4, how many of the times are above one standard deviations below the mean.
70, 75, 80, 85, 90, 105, 105, 130, 130, 130
a) two
b) three
c) four
d) five
e) one
Not "above" - that's an interpretation (and the exact false interpretation that whoever wrote the question wanted you to take). The original question reads "more than one standard deviation below the mean" - which means "take the mean, go down one SD, and see which of the times are at a distance of more than one SD from the mean"

Mean = 100
SD = 22.4
So you're looking for numbers that are far from the mean by more than 22.4: numbers smaller than 100-22.4 = 77.6
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