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Statistics

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by Apoorva@5 » Fri Jul 01, 2016 11:40 pm
The residents of Town X participated in a survey to determine the no. of hours per week each resident spent watching TV. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours.The no. of hours that Pat,A resident of Town X,watched TV last week was between 1&2 standard deviation below mean. so, the no. of hours he watched TV last week is?
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by Brent@GMATPrepNow » Sat Jul 02, 2016 5:47 am
When posting, please provide the ENTIRE question, including the answer choices.
Here's a link with a nice solution: https://www.beatthegmat.com/an-issue-reg ... 75194.html

Also, here's a video that covers the necessary concepts/skills to answer this question: https://www.gmatprepnow.com/module/gmat ... /video/809

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by [email protected] » Sat Jul 02, 2016 8:59 am
Hi Apoorva@5,

In the real world, Standard Deviation is based on a rather 'thick' math formula; thankfully, the GMAT will never ask you to use that formula or perform a standard deviation calculation that is all that complicated. SD is a rather rare subject on Test Day (you'll likely see it on just one question) and you'll be tested more on your general knowledge of the concept than on anything else.

In this particular prompt, we're given the mean (21 hours) and the SD (6 hours). We're asked for a value that is BETWEEN 1 and 2 SDs BELOW the mean. Since SD goes in both 'directions' from the mean, we would have the following results:

1 SD away would be (21-6) = 15 and (21+6) = 27
2 SDs away would be 21-12) = 9 and (21+12) = 33

Since we're specifically asked for BELOW the mean, we'd be looking for an answer that was between 9 and 15.

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by Matt@VeritasPrep » Sun Aug 06, 2017 10:47 pm
If the mean is 21 and the sd is 6, then

mean = 21
one sd below = 21 - 6 = 15
two sds below = 21 - 2*6 = 9

so the answer should be between 9 and 15.
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