GMAT PREP Std deviation

[warlock]

can anyone explain me this problem

thank you

[kiranlegend]

can anyone explain me this problem

thank you

is it A?

[warlock]

yes...what is the logic involved?

[kiranlegend]

yes...what is the logic involved?

A says that 30 % was removed from every tank.. so standard deviation remains the same and it will be 70% of 10 i.e., 7

with B we can't say anything as we don't know individual tank's volumes..

[4meonly]

In this case only average and median will change, SD will remain the same

[rishi235]

Hi....gotta doubt here with the actual value of the new SD....I know its not required in this problem but just wanna extrapolate

Heres how i approached it
V = Data Value
A = Average

S.D. = Sqrt{(V1-A)^2+(V2-A)^2+(V3-A)^2+(V4-A)^2+(V5-A)^2+(V6-A)^2}/6

Now, as the new values r 70% of the original....the average will also b 70% of the original
Let V' = New data values
& A' = New average value
So, V1' = 70% of V1, V2' = 70% of V2, etc....

So taking 70% common from the equation inside the sqrt,
We will have

New S.D.' = Sqrt (70%) of Old S.D.

Hence new SD=Sqrt(70) of 10...

Is this right...pls correct me if i'm wrong...

Thanks