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by sachindia » Fri Jan 04, 2013 5:53 am
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 meter. If the SD of ten running times is 22.4 seconds,rounded to 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

[spoiler]Ans : B[/spoiler]
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Sach
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Source: — Data Sufficiency |
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by Param800 » Fri Jan 04, 2013 6:33 am
(1) First calculate the mean of all the data given...which comes out to be 100

(2) Now, the standard deviation is 22.4, so one standard deviation below the mean would be 100-22.4 = 77.6.

(3)Now, we have to calculate the value corresponding with one standard deviation below the mean. We check the data points and find the points which are below 77.6. Only such points are 70 and 75

Thus, answer is 2 (B)

Hope this helps
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by sachindia » Fri Jan 04, 2013 6:50 am
but question is .. how many of the 10 running times are more than one SD below the mean of the 10 running times?

so it shud be 8, right?
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by Brent@GMATPrepNow » Fri Jan 04, 2013 7:14 am
sachindia wrote:but question is .. how many of the 10 running times are more than one SD below the mean of the 10 running times?

so it shud be 8, right?
The key word here is "below," which can be interpreted as "less than."
Similar sentence: "The sale price is more than $20 below the regular price."

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by sachindia » Fri Jan 04, 2013 8:11 am
Thanks Brent.. I get it now..
It is asking for the distribution points that lie in the region less than |m-d|

distribution usually is like below:

m-2d m-d d m+d m+2d ..

Please correct me if anything is wrong..
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by Brent@GMATPrepNow » Fri Jan 04, 2013 8:21 am
sachindia wrote:Thanks Brent.. I get it now..
It is asking for the distribution points that lie in the region less than |m-d|

distribution usually is like below:

m-2d m-d d m+d m+2d ..

Please correct me if anything is wrong..
Yes, that seems correct (I'm assuming that d = standard deviation)

Aside: we don't really need the absolute value (as in "|m-d|")

I'd say that we're looking for the distribution points that lie in the region less than m-d

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