Standard Deviation - Water Tank
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Source: Beat The GMAT — Data Sufficiency |
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
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
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samirpandeyit62
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