ans is 6 & 6. Why?

if you add 6 & 6 to the list the, avg does not change and adding the two #'s will actually make the std dev smaller because you have more #'s that are less from the mean(std dev).

Can someone explain this plz.

gmattester,

Take an eg of the set {2 4 6}
The mean will be 4.
The standard deviation can be found as [ (-2)^2 + 0^2 + (2)^2 /3 ]^1/2 (hope u get this step)

standard deviation = ( 8/3 ) ^ 1/2

When u add 4 and 4 to the set mentioned above ..

The standard deviation can be found as [ (-2)^2 + 0^2 + 0^2 + 0^2+ (2)^2 /5 ]^1/2

If u notice the denominator increases but numerator does not when 4 and 4 are added to {2 4 6}

Thanks Vignesh .......
Nice explanation.It's clear now