standard deviation (gmat prep q)

karenmeow · Posted: Sat Jul 19, 2008 11:18 am Post subject: standard deviation (gmat prep q)

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

the list shown consists of times, in seconds, that it took 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?

answer choices:
1
2
3
4
5

answer: 3

pepeprepa · Posted: Sat Jul 19, 2008 11:51 am Post subject:

Sum of all times: 1000
Mean of the 10 running times=100
1 standard deviation is 22.4 seconds, so the question asks you how many runners have a time which is superior to 122.4 seconds.
There are three times above (the three 130).

louvre · Posted: Wed Jul 23, 2008 6:49 am Post subject:

pepeprepa · Posted: Wed Jul 23, 2008 7:56 am Post subject:

Sorry for this concentration error, you are right.
We have 2 now

AleksandrM · Posted: Wed Jul 23, 2008 10:25 am Post subject:

My answer is also 2. The times sum to 1000 w/ a mean of 100

100 - 22.4 = 77.6 anything below that is more than 1 standard deviation below the mean.