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# The residents of Town X participated in a survey to

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Director
Joined: 17 Sep 2005
Posts: 903
The residents of Town X participated in a survey to [#permalink]

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09 Sep 2007, 16:32
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The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the result of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that John, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that John watched television last week?

1. 30
2. 20
3. 18
4. 12
5. 6

- Brajesh
CEO
Joined: 29 Mar 2007
Posts: 2559
Re: PS - Standard Deviation [#permalink]

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09 Sep 2007, 21:45
b14kumar wrote:
The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the result of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that John, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that John watched television last week?

1. 30
2. 20
3. 18
4. 12
5. 6

- Brajesh

21-6=15-6=9, So its btwn 15 and 9. Thus 12.

Ans D.
Senior Manager
Joined: 27 Aug 2007
Posts: 253

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09 Sep 2007, 22:49
21 - 6*1=15
21 - 6*2=9

9<x<15

Ans: D:12
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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09 Sep 2007, 22:51
1 SD below mean = 21-6 = 15
2 SD below mean = 21-12 = 9

So range = 9-15 hours. Only Choice D fits.
Director
Joined: 17 Sep 2005
Posts: 903

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10 Sep 2007, 06:18
Guys, can somebody please elaborate the concept/formula used here?

Thanks,
Brajesh
Manager
Joined: 29 Aug 2007
Posts: 110

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10 Sep 2007, 07:30
Fistail

You have applied a formulae here to calculate the SD.. Can you tell me what forumalae that is??
Director
Joined: 17 Sep 2005
Posts: 903

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11 Sep 2007, 22:34
Do we need to use some formula for this question?

- Brajesh
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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11 Sep 2007, 23:08
b14kumar wrote:
Do we need to use some formula for this question?

- Brajesh

Standard deviation curve is centered at the mean. Take a look at this plot and you will see what I mean...

http://en.wikipedia.org/wiki/Image:Standard_deviation_diagram.svg
Director
Joined: 03 May 2007
Posts: 872
Schools: University of Chicago, Wharton School

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11 Sep 2007, 23:28
b14kumar wrote:
Do we need to use some formula for this question?

- Brajesh

though there are formulas but the question clearly syas that "what is the value in between 1 and 2 sd"?
Director
Joined: 11 Jun 2007
Posts: 914

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11 Sep 2007, 23:43
also got D for the answer. The question is asking for a possible hour. We know from the question that the possible range is 1-2 SD below the mean of 21 hrs.

So the possible range is:

1 SD below: 21-6 = 15
2 SD below: 21-12 = 9

So between 9-15 hours. The only answer within this range is 12 (D).
Director
Joined: 17 Sep 2005
Posts: 903

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13 Sep 2007, 19:27
Thanks guys.

OA is D (12).

- Brajesh
13 Sep 2007, 19:27
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