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During an experiment, some water was removed from each of 6

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During an experiment, some water was removed from each of 6 [#permalink] New post 17 Jan 2007, 08:22
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During an experiment, some water was removed from each of 6 water tanks. If the standards deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment
(2)The average (arithmetic mean) volume of water in the tanks at the end of the experiement was 63 gallons
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 [#permalink] New post 17 Jan 2007, 10:16
My ANswer is A

SD still same after removing 30 percent from each tank
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 [#permalink] New post 17 Jan 2007, 10:30
My answer is A


St_Dev = SQRT[ ((a1- Avg)^2 + (a2 – Avg)^2 + …) / n]
Avg = (a1 + a2 + …) / n

From A we can calculate the new average and st. deviation:
Avg_new = (0.7*a1 + 0.7*a2 + …) / n = 0.7 * Avg

St_dev_new = SQRT[ ((0.7*a1 – 0.7*Avg)^2 + …) / n)]
= 0.7 * SQRT[ ((a1- Avg)^2 + (a2 – Avg)^2 + …) / n]
= 0.7 * St_dev

B is not sufficient
  [#permalink] 17 Jan 2007, 10:30
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During an experiment, some water was removed from each of 6

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