GMATPrep test 1 SD problem

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rainbownlife: Senior | Next Rank: 100 Posts; Posts: 37; Joined: Tue Jan 29, 2008 12:02 pm

GMATPrep test 1 SD problem

by rainbownlife » Tue Jan 29, 2008 12:15 pm

please explain. Waht are all the SD formule one needs to remmeber.

thanx in advance !

If a sample data has mean = 20 and std deviation=3.0 , which of the following is more than 2.5 std deviation from mean.

12,13.5,17,23.5,26.5

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Source: Beat The GMAT — Problem Solving | Tue Jan 29, 2008 12:15 pm

smushkas: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Sun Jul 01, 2007 5:56 pm; Thanked: 3 times

by smushkas » Tue Jan 29, 2008 2:04 pm

Hey there,

This is very easy one. All you have to do is just ad/subtract SD to/from the mean. Before you have to multiply by the given number, which is 2.5 SD. Thus,
20 + (2.5 * 3) = 20 + 7.5 = 27.5
20 - (2.5 * 3) = 20 - 7.5 = 12.5.
Your data should be outside the range of 12.5 ~ 27.5. All the numbers lie in the range except 12, the answer must be [A].

Last edited by smushkas on Tue Jan 29, 2008 2:14 pm, edited 1 time in total.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

Re: GMATPrep test 1 SD problem

by Stuart@KaplanGMAT » Tue Jan 29, 2008 2:09 pm

rainbownlife wrote:please explain. Waht are all the SD formule one needs to remmeber.

thanx in advance !

If a sample data has mean = 20 and std deviation=3.0 , which of the following is more than 2.5 std deviation from mean.

12,13.5,17,23.5,26.5

Smushka's explanation is 100% correct, I just want to add a strategic guessing tip for this question.

Since the question is asking us for the number outside of 2.5 SD, the only possible choices are the two extremes, (a) and (e) - so, even if you're stuck, you have a 50/50 shot at the question.