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SD Problem - GMATPrep

by queenisabella » Sat Mar 31, 2012 5:34 pm
70, 75, 80, 85, 90, 105, 105, 130, 130, 130
The list shown consists of the times, in seconds, that it took each of the 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 teams?
A) 1
B) 2
C) 3
D) 4
E) 5

OA is B
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by Bill@VeritasPrep » Sat Mar 31, 2012 5:44 pm
The standard deviation tells us distance from the mean, so we need to find the mean. The sum of the 10 times is 1000, so the mean is 100.

1 standard deviation below the mean is 100 - 22.4 = 77.6

Only 70 and 75 are below 77.6
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