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Standard Deviation

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by okigbo » Wed Oct 28, 2009 6:59 pm
Some water was removed from each of 6 tanks. If standard deviation of the volumes of water at the beginning was 10 gallons, what was the standard deviation of the volumes at the end? A
a. For each tank, 30% of water at the beginning was removed
b. The average volume of water in the tanks at the end was 63 gallons
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Source: — Data Sufficiency |
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by life is a test » Wed Oct 28, 2009 11:32 pm
the s.d. is the distance of each of the values from the avg.

in order to impact the s.d. we need to know the actual amount taken away from each of the cylinders or the percentage of water taken away from each cylinder (i.e. the % decrease in the avg) which A does.

B insuff since we dont know the value of avg before the reduction hence cannot compute a % decrease in avg.

hope this helps.
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by jaspreet_takhar » Thu Oct 29, 2009 12:53 am
Agree with "life is a test"

Also, I think if you reduce the quantity by an equal percentage from all the containers, the SD would remain the same...
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by life is a test » Thu Oct 29, 2009 7:29 pm
jaspreet_takhar wrote:Agree with "life is a test"

Also, I think if you reduce the quantity by an equal percentage from all the containers, the SD would remain the same...
only if you reduce it by the same amount not percentage. reducing it by the same percentage would lead to the sd reducing by the same percentage hence in this case A tells us the sd would reduce by 30% i.e. would go down to 7.
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