## SD Problem - GMATPrep

##### This topic has expert replies
Senior | Next Rank: 100 Posts
Posts: 47
Joined: 05 Feb 2012
Followed by:3 members

### 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
Source: GMATPrep

GMAT Instructor
Posts: 1248
Joined: 29 Mar 2012
Location: Everywhere
Thanked: 503 times
Followed by:191 members
GMAT Score:780
by [email protected] » 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
Join Veritas Prep's 2010 Instructor of the Year, Matt Douglas for GMATT Mondays

Visit the Veritas Prep Blog

Try the FREE Veritas Prep Practice Test

• Page 1 of 1