GMAT Question Bank | Number Properties DS Q28

This GMAT data insights practice question is a data sufficiency question from Number Theory. Concept: properties of positive and negative numbers. It is a medium difficulty GMAT 625 level DI 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 28: Is ab negative?

1. (a + b)² > (a - b)²
2. a = b

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Video Explanation

Play Video: GMAT Data Insights Practice Question

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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?
The question is an "Is" question. Answer to an "is" question is either YES or NO.

When is the data sufficient?
The data is sufficient if we are able to get a DEFINITE YES or a DEFINITE NO from the information given in the statements.

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Step 2 of solving this GMAT DS question:
Evaluate Statement (1) ALONE: (a + b)² > (a - b)²

Expanding both sides of the inequality, we get a² + b² + 2ab > a² + b² - 2ab
Simplifying we get, 4ab > 0 or ab > 0.
So, we can conclude that ab is NOT negative. We have got a definite NO as the answer.

Statement 1 ALONE is sufficient.
Eliminate choices B, C, and E. Choices narrow down to A or D.

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Step 3 of solving this GMAT DS question:
Evaluate Statement (2) ALONE: a = b

This is actually the statement that could trick you.
a = b.
So, either both a and b or positive or both a and b are negative. In either case ab is positive.
There is a third possibility: a and b can both be 0. In that scenario, ab = 0

The catch is that, when we see that ab could be positive or ab could be zero, we may be tempted to think that we do not have a conclusive answer from this statement.

Let's take both the scenarios and find the answer to our question "Is ab negative?"
If ab is positive, the answer to the question is NO.
If ab is 0, the answer to the question is NO.
We are able to conclude that ab is NOT negative with statement 2 alone.

Statement 2 ALONE is ALSO sufficient.
Eliminate choice A.

Each statement is INDEPENDENTLY sufficient to answer the question. Choice D is the answer.