Standard Deviation - Water Tank

Can anyone slove the above question pls??

I found the same question on testmagic and one of the guys there has given the following explaination, May be Samir Pandey or Gabriel can give their comments as well? But for now here is the explaination I found.

Ok guys.. many people have guessed it right but with wrong reason ... A is the answer but that doesn't mean that SD will be the same.. no it will be multiply by 0.7 therefore the final SD will be 7.. Confused... I tell you how..

Lets say water in 6 tanks are a,b,c,d,e,f respectively..

Mean would be a + b + c+ d+ e + f / 6 == M (say)

and SD = SQRT ( (a-M)^2 + (b-M)^2 + ....)/6 = N say

Ok lets say now when 30% water is removed

So mean would be 0.7a + 0.7b + 0.7c + 0.7d + 0.7e + 0.7f /6 = 0.7M

Now SD = SQRT ( (0.7a-0.7M)^2 + (0.7b - 0.7M)^2 + ... /6
= 0.7 * N = 0.7 * 10 = 7

SO answer is A

Great explanation ri2007!!Thanks

hey Samir

Thanks for posting the link. Your explaination are always one of the best I have seen any where.