Standard Deviation - Water Tank

gmatrant · Tue Nov 13, 2007 8:37 am

Just want to know the approach to solve this
OA is A

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moneyman · Fri Nov 16, 2007 8:44 pm

Can anyone slove the above question pls??

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ri2007 · Fri Nov 16, 2007 8:59 pm

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