Official Guide Explanation:
Problem Solving #54

Background

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: 54
Page: 159
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Powers and Roots of Numbers > Roots

Explanation: Don't add 80 and 125! That's the trap the GMAT is setting for you.

Instead, simplify each of the radicals. Find factors of 80 and 125 that are squares. For instance, 80 = 16(5), so rt[80] = 4 rt[5]. 125 = 25(5), so rt[125] = 5 rt[5].

Add them together:

4 rt[5] + 5 rt[5] = 9 rt[5], choice (A).

Click here for the full list of GMAT OG12 explanations.

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.