Bookshelf

 Total GMAT Math Jeff's complete Quant guide, on sale now!
 Total GMAT Verbal Everything you need to ace GMAT Verbal!
 New: GMAT 111 Improve every aspect of your GMAT prep!

1,800 Practice Math Questions

GMAT Official Guide
OG Math | OG Verbal

Guides To the Official Guide
Free: OG12 explanations!

GMAT Question of the Day

Beginner's Guide to the GMAT

GMAT Hacks Affiliate Program

Resources

MBA.com
GMAC Official Site
Free GMATPrep Practice Tests

Stacy Blackman Consulting
Book | Essay Guides

GRE HQ
Total GRE Math

Ultimate SAT Verbal

Official Guide Explanation:Data Sufficiency #26

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.

Question: 26
Page: 274
Difficulty: 6 (Moderately Difficult)
Category 1: Algebra > Equations >
Category 2: Algebra > Functions > other

Explanation: While the given formula looks complicated, there are only three variables. Even better, c is a constant, so when we solve for it once, we'll never need to solve for it again.

Statement (1) is sufficient. The information we're given, combined with the explanation in the question, tells us that t = 1 (the month) and n = 180 (the number of units). Start with the given formula and then plug in the values:

n = (900/(1 + c2-t))

180 = (900/(1 + c(2-1)))

I don't recommend spending the time to solve for c, but recognize that you could solve for c in that equation. Once you have c, a constant, the formula only has two variables. The question is asking how many units (n) are product in month 6--in other words, what is the value of n when t = 6? Since we know t and we know c, n is the only variable, meaning we can solve for it.

Statment (2) is also sufficient. The same reasoning applies as in (1). In this case, we just plug different values of n and t into the equation to find the constant c:

300 = (900/(1 + c(2-2)))

Again, no reason to solve, but you can find c. That's enough to find the value of n when t = 6. Choice (D) is correct.

 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.