SD good Q

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anksgupta: Senior | Next Rank: 100 Posts; Posts: 51; Joined: Wed Apr 29, 2009 9:00 am; Thanked: 6 times

by anksgupta » Fri Jun 26, 2009 6:04 am

A set of data consists of the following 5 numbers : {0,2,4,6,8}

What 2 no.s if added to create a set of 7, will result in SD that is closest to the SD for set of 5 original nos.

1. -1,9
2. 4,4
3. 3,5
4. 2,6
5. 0,8

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Source: Beat The GMAT — Problem Solving | Fri Jun 26, 2009 6:04 am

DanaJ: Site Admin; Posts: 2567; Joined: Thu Jan 01, 2009 10:05 amFollowed by: 550 members; Thanked: 712 times; GMAT Score:770

by DanaJ » Fri Jun 26, 2009 7:53 am

A quick look at the set you provided allows us to determine that the average of the set is 4.
The SD is actually a measurement of the average distance between the numbers of a set and the average of said set.
In this case, the SD is around 2. something. This is because the distance between 4 and (2 and 6) is 2, while the distance between 4 and (0 and 8) is 4. The average distance should be something like 3, but since we also have a 4 in the set (which is also the average - so the distance between 4 and 4 is zero), the SD is tilted towards 2 a little.

So you're basically looking for the set of numbers that are kind of 2 points away from the average and your best bet would be, IMHO, 2 and 6.

I may be wrong though, since statistics is not my favorite field.

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ssmiles08: Master | Next Rank: 500 Posts; Posts: 472; Joined: Sun Mar 29, 2009 6:54 pm; Thanked: 56 times

by ssmiles08 » Fri Jun 26, 2009 12:14 pm

wow these kind of questions have me terrified

Do these types have a realistic chance of appearing on the GMAT...the only reason I ask so is because it seems like you sort of have to find the actual standard deviation...unless there is some mystery method involved i haven't figured out yet??

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Claret: Senior | Next Rank: 100 Posts; Posts: 97; Joined: Sat Jun 06, 2009 10:48 pm; Thanked: 4 times

by Claret » Fri Jun 26, 2009 12:23 pm

Yes!! These questions have a fairly decent chance of appearing on the GMAT.

Infact i came across 2 SD questions on the gmat both on similar lines as the above question!!

I even remember the questions but revealing them would be against the gmac non-disclosure agreement !!

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ssmiles08: Master | Next Rank: 500 Posts; Posts: 472; Joined: Sun Mar 29, 2009 6:54 pm; Thanked: 56 times

by ssmiles08 » Fri Jun 26, 2009 12:26 pm

Claret wrote:Yes!! These questions have a fairly decent chance of appearing on the GMAT.

Infact i came across 2 SD questions on the gmat both on similar lines as the above question!!

I even remember the questions but revealing them would be against the gmac non-disclosure agreement !!

oh boy...fantastic (obvious sarcasm)........better get cracking then! Thanks Claret! Congrats on your GMAT score btw! it is still pretty darn good

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Claret: Senior | Next Rank: 100 Posts; Posts: 97; Joined: Sat Jun 06, 2009 10:48 pm; Thanked: 4 times

by Claret » Fri Jun 26, 2009 12:31 pm

thanks ssmiles08 ! i m going to re-take the test though !!

btw when is ur test ?

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ssmiles08: Master | Next Rank: 500 Posts; Posts: 472; Joined: Sun Mar 29, 2009 6:54 pm; Thanked: 56 times

by ssmiles08 » Fri Jun 26, 2009 12:39 pm

Claret wrote:thanks ssmiles08 ! i m going to re-take the test though !!

btw when is ur test ?

I am hoping for sometime in July...haven't registered yet...I have just started my verbal prep...so that will have to run its course and we'll see

apologies to anksgupta, for hijacking the thread for a little bit...hopefully you don't mind!

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niraj_a: Legendary Member; Posts: 708; Joined: Sun Jun 01, 2008 4:59 amFollowed by: 1 member; Location: USA; Thanked: 13 times

by niraj_a » Fri Jun 26, 2009 12:39 pm

im thinking its 4 and 4 because since the difference between the remaining numbers i.e. 2 wont change this way, the SD wont change either.

did i get it right?

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Claret: Senior | Next Rank: 100 Posts; Posts: 97; Joined: Sat Jun 06, 2009 10:48 pm; Thanked: 4 times

by Claret » Fri Jun 26, 2009 12:48 pm

ssmiles08 wrote:
I am hoping for sometime in July...haven't registered yet...I have just started my verbal prep...so that will have to run its course and we'll see

Seeing your detailed explanations of math questions makes me think u ll definitely crack the test!!

All the very best

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anksgupta: Senior | Next Rank: 100 Posts; Posts: 51; Joined: Wed Apr 29, 2009 9:00 am; Thanked: 6 times

by anksgupta » Fri Jun 26, 2009 12:53 pm

OA is D

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