Series

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MBA.Aspirant: Master | Next Rank: 500 Posts; Posts: 401; Joined: Tue May 24, 2011 1:14 amFollowed by: 5 members; Thanked: 37 times

by MBA.Aspirant » Wed Jun 22, 2011 9:34 am

In a given series 3, -1, 2, 1, if the difference between 10th number and the 7th number of the series is 'x' then what's the value of x?

I don't understand how to get the difference here..

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Source: Beat The GMAT — Problem Solving | Wed Jun 22, 2011 9:34 am

Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 amFollowed by: 53 members; Location: India; Thanked: 375 times

by Frankenstein » Wed Jun 22, 2011 9:42 am

Hi,
This is definitely a bad question. No where the series constraints have been stated. But, just by looking the numbers, I feel the series is a Fibonacci type with each term equal to sum of two preceding terms.
t5 = t3+ t4 = 2+1 =3
similarly, t6 = 1+3 =4
t7 = 3+4 =7
t8 = 4+7 =11
t9 = 7+11 =18
t10 = 11+18 =29
x = t10- t7 = 29-7 =22

Cheers!

Things are not what they appear to be... nor are they otherwise

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MBA.Aspirant: Master | Next Rank: 500 Posts; Posts: 401; Joined: Tue May 24, 2011 1:14 amFollowed by: 5 members; Thanked: 37 times

by MBA.Aspirant » Wed Jun 22, 2011 9:54 am

Thanks alot man. Are we supposed to know the types of series other than arithmetic and geometric prog? It's usually stated in the q that this is a fibonacci series

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Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 amFollowed by: 53 members; Location: India; Thanked: 375 times

by Frankenstein » Wed Jun 22, 2011 9:59 am

Hi,
I don't think we need to know about it. It will be stated in GMAT questions.

Cheers!

Things are not what they appear to be... nor are they otherwise

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Wed Jun 22, 2011 12:41 pm

Yes, Frankenstein is right - you have *no* information in the question about how the sequence progresses beyond the first four terms, so it is absolutely impossible to answer the question. I suspect that Frankenstein is right, and the question means to ask about a Fibonacci type sequence - something you don't need to know about for the GMAT - but the question absolutely needs to make clear what rule is being used to generate the terms in the sequence. They're also talking about a sequence in the question, and not a "series", so in addition to designing impossible questions, they're not even using mathematical terminology correctly. Where is the question from?

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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edvhou812: Master | Next Rank: 500 Posts; Posts: 143; Joined: Sat Nov 06, 2010 9:06 pmFollowed by: 1 member; Thanked: 10 times

by edvhou812 » Wed Jun 22, 2011 7:13 pm

Ian Stewart wrote:Yes, Frankenstein is right - you have *no* information in the question about how the sequence progresses beyond the first four terms, so it is absolutely impossible to answer the question. I suspect that Frankenstein is right, and the question means to ask about a Fibonacci type sequence - something you don't need to know about for the GMAT - but the question absolutely needs to make clear what rule is being used to generate the terms in the sequence. They're also talking about a sequence in the question, and not a "series", so in addition to designing impossible questions, they're not even using mathematical terminology correctly. Where is the question from?

Is it possible to have the moderator delete "bad" questions from the site? This is the third "bad" question I've seen in the past 24 hours.

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