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Standard Deviation

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by raju232007 » Sat Oct 25, 2008 6:30 am
A set of data consists of the following 5 numbers: 0,2,4,6 and 8.Which two numbers,if added to create a set of 7 numbers will result in a new standard deviation that is close to the standard deviation for the original 5 numbers?
A.-1 and 9
B. 4 and 4
C. 3 and 5
D. 2 and 6
E. 0 and 8


OA is D


I have already posted this question sometime back in this forum..But I still have my doubts on whether this problem can be solved using a shorter method without the application of formulas....I kindly request the experts to suggest the best approach to tackle this problem..
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by rohangupta83 » Sun Oct 26, 2008 1:04 am
I could do it only by calculating standard deviation individually

That way it's too long - if there is a better way, please let me know
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by schumi_gmat » Sun Oct 26, 2008 11:47 am
SD of 5 numbers is sqrt(40/7) = sqrt(8)

only way we can get 8 again is by adding 0 and 8, i.e 40+16)/7 = 8
hence IMO E
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by stop@800 » Sun Oct 26, 2008 2:18 pm
schumi_gmat wrote:SD of 5 numbers is sqrt(40/7) = sqrt(8)

only way we can get 8 again is by adding 0 and 8, i.e 40+16)/7 = 8
hence IMO E
Actually all the numbers adds to 8 :)

we are waiting for Ian or some other expert to comment on.
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