Arithmetic Progression

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savary: Newbie | Next Rank: 10 Posts; Posts: 8; Joined: Mon Aug 03, 2015 1:32 am; Location: Egypt

Arithmetic Progression

by savary » Wed Nov 11, 2015 6:28 am

I came across this question on https://www.questionbank.4gmat.com.

Data Sufficiency : A set S contains the following elements: {7, 11, 15, 19, 23, x}. What is the value of x?

1. The elements are in arithmetic progression.
2. x is prime.

I assumed X has to be 27 since it comes after 23 in the arithmetic sequence. But can X be 3? What is the proper definition for "arithmetic progression".

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Source: Beat The GMAT — Data Sufficiency | Wed Nov 11, 2015 6:28 am

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Wed Nov 11, 2015 6:53 am

savary wrote:I came across this question on https://www.questionbank.4gmat.com.

Data Sufficiency : A set S contains the following elements: {7, 11, 15, 19, 23, x}. What is the value of x?

1. The elements are in arithmetic progression.
2. x is prime.

I assumed X has to be 27 since it comes after 23 in the arithmetic sequence. But can X be 3? What is the proper definition for "arithmetic progression".

Bad question. The term "arithmetic progression" appears nowhere in the Official Guide.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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by [email protected] » Wed Nov 11, 2015 9:56 am

Hi savary,

Brent is correct; the GMAT has never presented the concept of an arithmetic sequence using this type of language. You MIGHT see a question involving an arithmetic sequence on Test Day, but the question-writer would define what an arithmetic sequence actually is before asking you to work with one.

If this source isn't offering proper GMAT practice materials, then you might want to invest in a different resource.

To answer your question though, to fit the arithmetic sequence that is implied by the set of numbers, X would have to be either 3 or 27.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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Jim@StratusPrep: MBA Admissions Consultant; Posts: 2279; Joined: Fri Nov 11, 2011 7:51 am; Location: New York; Thanked: 660 times; Followed by:266 members; GMAT Score:770

by Jim@StratusPrep » Wed Nov 11, 2015 11:54 am

Also, recognize that you will never get contradictory information from the statements. If x = 27 from the 'bad wording', it cannot be prime in statement 2.

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by Matt@VeritasPrep » Fri Nov 13, 2015 3:54 pm

Brent@GMATPrepNow wrote:Bad question. The term "arithmetic progression" appears nowhere in the Official Guide.

They use the analogous term "arithmetic sequence", however (pg. 174) and they use that on a question in GMATPrep as well. The problem is that they define the term for the reader in each question, so they don't seem to assume that you already know what an AS/AP is.

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Fri Nov 13, 2015 3:57 pm

Matt@VeritasPrep wrote:
Brent@GMATPrepNow wrote:Bad question. The term "arithmetic progression" appears nowhere in the Official Guide.
They use the analogous term "arithmetic sequence", however (pg. 174) and they use that on a question in GMATPrep as well. The problem is that they define the term for the reader in each question, so they don't seem to assume that you already know what an AS/AP is.

I think the key point here is the test-makers will not assume that the test-taker knows the names of any sequences. Instead, they will define the terms in the sequence.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com