BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

standard deviation

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 226
Joined: Thu Nov 25, 2010 12:19 am
Thanked: 3 times
Followed by:2 members

standard deviation

by nafiul9090 » Tue Aug 30, 2011 12:22 am
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


please shed some light on this problem
Top
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 516
Joined: Fri Jul 31, 2009 3:22 pm
Thanked: 112 times
Followed by:13 members

by smackmartine » Tue Aug 30, 2011 12:44 am
IMO B
Mean(70, 75, 80, 85, 90, 105, 105, 130, 130, 130 ) =1000/10 = 100

1 Standard deviation from Mean means 100+22.4=122.4 and 100-22.4=77.6

Because we are concerned about the times those are more than 1 standard deviation below the mean, only values which suffices this conditions are 70 and 75 (We do not care values greater than Mean). On number line :

|--------|----|----1 std dev--------|--------1 std dev---------|---
70____75__77.6___________100________________122.4
Smack is Back ...
It takes time and effort to explain, so if my comment helped you please press Thanks button :)
Top
Post new topic Post Reply
• Page 1 of 1