Arithmetic Progression

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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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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.

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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.

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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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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.

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by savary » Sun Nov 15, 2015 1:10 am
Thank you all for the explanation. It is crystal clear now :)