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GMAT prep question 2

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by willymtz » Wed May 02, 2007 10:43 am
If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than 2.5 standard deviations from the mean?

a) 12.0
b) 13.5
c) 17.0
d) 23.5
e) 26.5
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by Cybermusings » Wed May 02, 2007 10:45 am
Is it A?

Mean = 20
SD = 3
2.5 SD = 2.5*3 = 7.5

Only A fits the bill
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by willymtz » Wed May 02, 2007 10:54 am
Yes it is A. Thanks for the reply. It was a really easy question. Didnt quite understand it when I first read it.

Thanks.
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by weipei » Fri May 04, 2007 7:35 pm
Can you explain little bit more on the answer? Thanks!
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by chrsells » Fri May 04, 2007 10:23 pm
weipei wrote:Can you explain little bit more on the answer? Thanks!
This might be represented algebraically as |X-20| ≥ 7.5

When given X, the value on the left side of the equation will yield some difference between X and the mean. That difference must be greater than 2.5 times the standard deviation of 3.0 - in other words, 7.5.

At this point it's only a matter of solving for values that satisfy the equation. (A) serves up the only number that qualifies.
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