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Standard Deviation Problem on GMAT Prep

Expert replies
by wcheng57 » Thu Jul 22, 2010 11:59 pm
I came across this problem during a GMAT Prep exam. No explanations provided, only the correct answer. Please explain how you arrived at the answer. Thanks.

The distribution of the results of a survey had a mean of 21 hours and a standard deviation of 6 hours. The # of hours that Pat watched TV last week was between 1 and 2 standard deviation below the mean. Which of the following could be the # of hours Pat spent watching TV?

a) 30
b) 20
c) 18
d) 12
e) 6

D is correct answer.
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Source: — Problem Solving |

by outreach » Fri Jul 23, 2010 12:06 am
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by selango » Fri Jul 23, 2010 12:06 am
Mean=21

SD=6

1 SD below the mean=21-6=15

2 SD below the mean=21-12=9

between 9 and 15

12 only satisfy the above.

Hence D

For SD and Mean prob like this you can use the below formula,


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