The GMAT DS question given below is from the topic Arithmetic Progressions. Problems on Arithmetic Progressions are quite easy if one understands that the basic concept behind AP is just an extrapolation of simple multiplication table of the common difference of the series. It is a medium difficulty GMAT 600 to 650 level data sufficiency question.

This data sufficiency problem consists of a question and two statements, labeled (1) and (2), in which certain data are given. You have to decide whether the data given in the statements are sufficient for answering the question. Using the data given in the statements, plus your knowledge of mathematics and everyday facts (such as the number of days in a leap year or the meaning of the word counterclockwise), you must indicate whether -

- Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the question asked.
- Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the question asked.
- BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but NEITHER statement ALONE is sufficient to answer the question asked.
- EACH statement ALONE is sufficient to answer the question asked.
- Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed.

All numbers used are real numbers.

A figure accompanying a data sufficiency question will conform to the information given in the question but will not necessarily conform to the additional information given in statements (1) and (2)

Lines shown as straight can be assumed to be straight and lines that appear jagged can also be assumed to be straight

You may assume that the positions of points, angles, regions, etc. exist in the order shown and that angle measures are greater than zero.

All figures lie in a plane unless otherwise indicated.

In data sufficiency problems that ask for the value of a quantity, the data given in the statement are sufficient only when it is possible to determine exactly one numerical value for the quantity.

Question 22: A set S contains the following elements: {7, 11, 15, 19, 23, x}. What is the value of x?

- The elements are in arithmetic progression
- x is prime

**What kind of an answer will the question fetch?**

The question is a "What is the value?" question. For questions asking for a value, the answer should be a number.

**When is the data sufficient?**

The data is sufficient if we are able to get a UNIQUE answer for the value of 'x' from the information in the statements.

If either the statements do not have adequate data to determine the value of 'x' or if more than one value of 'x' exists based on the information in the statement, the data is NOT sufficient.

Evaluate Statement (1) ALONE: The elements are in arithmetic progression

The common difference of the sequence is 4.

So, x could either be 3 or 27.

We are not able to find a UNIQUE value for x from statement 1.

**Statement 1 ALONE is NOT sufficient.**

Eliminate choices A and D. Choices narrow down to B, C, or E.

Evaluate Statement (2) ALONE: x is prime

x could be any prime number. There are infinite possibilities.

We are not able to find a UNIQUE value for x from statement 2.

**Statement 2 ALONE is NOT sufficient.**

Eliminate choice B. Choices narrow down to C or E.

Evaluate Statements (1) & (2) Together: The elements are in arithmetic progression & x is prime

From statement 1 we have narrowed down the values that 'x' could take to 3 or 27;

From statement 2 we know that x is prime.

3 is the only value that satisfies both the conditions.

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