SD question

veekay · Posted: Sat Aug 25, 2007 5:35 pm Post subject: SD question

Source: Gmat MATH tough problems

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 after few replies

beny · Posted: Sat Aug 25, 2007 6:06 pm Post subject:

veekay · Posted: Sun Aug 26, 2007 8:46 am Post subject:

givemeanid · Posted: Sun Aug 26, 2007 10:51 am Post subject:

SD = sqrt of (sum of difference betn mean and each / N)
= sqrt (sum (x-mean) / N)
= sqrt (V) [Lets call it V]

For 0,2,4,6,8, the mean is 4.
So, V = 16+4+0+4+16 / 5 = 8

Now, after adding two numbers, N = 7
So, to keep V same (or close), numerator in V should be 56 or close to it.

A. adds 25+25 = 50
B. adds 0+0 = 0
C. adds 1+1 = 2
D. adds 4+4 = 8
C. adds 16+16 = 32

D comes to 48 which is the closest to 56. It is D.

veekay · Posted: Mon Aug 27, 2007 8:55 am Post subject: