GMAT Question of the Week: Data Sufficiency and Averages – The Explanation

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GMAT Question of the Week: Data Sufficiency and Averages - The Explanation

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If you haven't already, visit our Data Sufficiency practice problem and give it a try on your own before reading the explanation.

To get this question correct, you must combine your knowledge of fundamental math concepts with use of the Kaplan Method and strategies for approaching Data Sufficiency. Here’s a breakdown:

The average formula is Average = Sum of the terms / Number of terms.

The average of m and n is (m + n) / 2  . The question stem says “Is  (m + n) / 2  < 50 ?”

Remember, with a Yes/No Data Sufficiency question, you are looking at the statements and trying to determine whether they provide a consistent YES or NO answer to this question. A consistent answer of yes OR no is sufficient. An inconsistent answer (yes and no) is insufficient.

Statement (1): Sufficient. This statement says that (3m + 3n) / 2 = 90.

Pull the 3 out of the numerator to get 3(m + n) / 2 = 90,

Multiply both sides by 2 to get 3(m + n) = 180,

Then divide both sides by 3 to get m + n = 60.

So, the average of m and n is  =  = 30. The average of m and n is definitely less than 50, and the answer to the question in the stem is always “yes.” Statement (1) is sufficient, so you can eliminate (B), (C), and (E).

Statement (2): Insufficient. This does not allow us to determine the average of m and n, because it does not give us the values or the right relationship between m and n. Eliminate (D).

Answer choice (A) is correct.

 

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