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In the last two years, each of Jeremy’s six children grew in

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In the last two years, each of Jeremy’s six children grew in [#permalink] New post 12 Apr 2012, 15:05
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In the last two years, each of Jeremy’s six children grew in height by at least one inch. If the standard deviation of their heights two years ago was 4.5 inches, what is the standard deviation of their heights?

(1) In the last two years, the heights of Jeremy’s six children have increased a total of 17 inches.
(2) In the last two years, each child’s height has increased by 5 percent.

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Re: In the last two years, each of Jeremy’s six children grew in [#permalink] New post 12 Apr 2012, 16:25
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In the last two years, each of Jeremy’s six children grew in height by at least one inch. If the standard deviation of their heights two years ago was 4.5 inches, what is the standard deviation of their heights?

We are given a set with 6 terms and the standard deviation of 4.5. Next, we are told that each term of this set was increased and are asked to find the new standard deviation of this changed set.

(1) In the last two years, the heights of Jeremy’s six children have increased a total of 17 inches. Not sufficient, since many different scenarios of increase of each data point are possible.

(2) In the last two years, each child’s height has increased by 5 percent. Important property: if we increase or decrease each term in a set by the same percent (multiply all terms by the constant): SD will increase or decrease by the same percent. So, since each of 6 terms has increased by 5% then the standard deviation of 4.5 would also increase by 5% and become 4.5*1.05. Sufficient.

Answer: B.
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Re: In the last two years, each of Jeremy’s six children grew in   [#permalink] 12 Apr 2012, 16:25
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