Official Guide Explanation:
Problem Solving #90




This is just one of hundreds of free explanations I've created to the quantitative questions in The Official Guide for GMAT Quantitative Review (2nd 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: 90
Page: 73
Difficulty: 5 (Moderate)
Category 1: Arithmetic > Powers and Roots of Numbers > Powers
Category 2: Word Problems > Rate Problems > other

Explanation: To keep things simple, ignore the thousands: pretend that the question asks how many minutes it takes for the population to grow from 1 to 500. To find how many times it must double, start multiplying by 2. 1(2) = 2, 2(2) = 4, 4(2) = 8, 8(2) = 16, 16(2) = 32, 32(2) = 64, 64(2) = 128, 128(2) = 256, 256(2) = 512. Altogether, that's 9 doublings, so at two minutes per doubling, our answer is 2(9) = 18, choice (E). If you know your powers of 2, this is very easy math; if you've memorized what the powers are, you should be able to instantly recognize that 500 is near 512, which is 29, thus saving you plenty of time and calculation.

Click here for the full list of GMAT Quant Review 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.