GMAT Maths | Arithmetic Progressions Q24

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 tables. It is a medium difficulty GMAT 600 to 650 level data sufficiency question.

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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 -

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

Numbers

All numbers used are real numbers.

Figures

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.

Note

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 24: What is the 6^th term of the Arithmetic sequence?

1. The sum of the 6^th to the 12^th term of the sequence is 77.
2. The sum of the 2^nd to the 10^th term of the sequence is 108.

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Video Explanation

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Explanatory Answer

Step 1 of solving this GMAT DS question: Understand the Question Stem

What kind of an answer will the question fetch?
We have to find the 6^th term of an arithmetic sequence. The answer has to 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 6th term of the sequence from the information in the statements.
If either the statements do not have adequate data to determine the value of 6th term or if more than one value exists based on the information in the statement, the data is NOT sufficient.

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Step 2 of solving this GMAT DS question:
Evaluate Statement (1) ALONE: The sum of the 6^th to the 12^th term is 77

Using the sum upto n terms formula we get 77 = \\frac {7} {2})(a₆ + a₁₂) where a₆ is the 6^th term and a₁₂ is the 12^th term.

Simplifying the expression, we get 22 = a₆ + a₁₂ ..... equation (1)
But a₆ = a₁ + 5d and a₁₂ = a₁ + 11d
So, we can write equation (1) as a₁ + 5d + a₁ + 11d = 22
Or 2a₁ + 16d = 22
or a₁ + 8d = a₉ = 11
From this statement, we can determine that a₉ = 11. However, we will not be able to find the value of the 6^th term.

Statement 1 ALONE is NOT sufficient.
Eliminate choices A and D. Choices narrow down to B, C, or E.

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Step 3 of solving this GMAT DS question:
Evaluate Statement (2) ALONE: The sum of the 2^nd to the 10^th term of the sequence is 108

Using the sum upto n terms formula we get 108 = \\frac {9} {2})(a₂ + a₁₀) where a₂ is the 2^nd term and a₁₀ is the 10^th term of the sequence.

Simplifying the equation, we get 24 = a₂ + a₁₀
But, a₂ = a₁ + d and a₁₀ = a₁ + 9d
So, 24 = a₁ + d + a₁ + 9d
or 24 = 2a₁ + 10d
or 12 = a₁ + 5d
But a₁ + 5d = a₆ = 12.
Hence, from statement 2 we can determine the value of a₆.

Statement 2 ALONE is sufficient. Choice B is the answer.