Official Guide Explanation:
Data Sufficiency #137

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: 137
Page: 285
Difficulty: 4 (Moderately Easy)
Category 1: Algebra > Linear Equations-Two Unk >

Explanation: If the price of a donut is d and the price of a bagel is b, we're looking for 5d + 3b.

Statement (1) is sufficient. It gives us an equation:

10d + 6b = 12.90

That's one equation with two variables, but notice that the ratio of 10 to 6 is the same as that of 5 to 3. Divide both sides of the equation by 2 to make it look even more similar:

5d + 3b = 6.45

There's your answer.

Statement (2) is insufficient. Another equation:

d = b - 0.15

This time, we can't manipulate the equation to find the value of 5d + 3b. We could substitute b - 0.15 for d, but that would leave us with an expression with a variable remaining:

5(b - 0.15) + 3b

Choice (A) is correct.

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.