Official Guide Explanation:
Data Sufficiency #42




This is just one of hundreds of free explanations I've created to the quantitative questions in The Official Guide for GMAT Review (12th ed.). Click the links on the question number, difficulty level, and categories to find explanations for other problems.

These are the same explanations that are featured in my "Guides to the Official Guide" PDF booklets. However, because of the limitations of HTML and cross-browser compatibility, some mathematical concepts, such as fractions and roots, do not display as clearly online.

Click here for an example of the PDF booklets. Click here to purchase a PDF copy.


Solution and Metadata

Question: 42
Page: 276
Difficulty: 6 (Moderately Difficult)
Category 1: Word Problems > Geometry Problems >
Category 2: Geometry > Circles > other
Category 3: Arithmetic > Ratio and Proportion >

Explanation: The real - life scenario of the water pipe makes this question look more complicated than it is. We've got two circles with the same center; the smaller one has a radius of r; the larger has a radius of t.

Statement (1) is sufficient. We're told that ((t - r)/(r)) = 0.5 and t - r = 0.3. So combining them, we get:

((0.3)/(r)) = 0.5

That's enough to find r. (No need on DS to worry about handling the decimals.)

Statement (2) is insufficient. The area of the cross section is the difference between the area of the larger circle and the area of the smaller circle, or:

π t2- π r2 = 1.29 π

That's not enough to find a value for r. Choice (A) is correct.

Click here for the full list of GMAT OG12 explanations.


You should follow me on Twitter. While you're at it, take a moment to subscribe to GMAT Hacks via RSS or Email.

Total GMAT Math

The comprehensive guide to the GMAT Quant section. It's "far and away the best study material available," including over 300 realistic practice questions and more than 500 exercises!
Click to read more.