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## Official Guide Explanation:Data Sufficiency #13

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: 13
Page: 273
Difficulty: 3 (Very Easy)
Category 1: Algebra > Linear Equations-One Unk >
Category 2: Word Problems > Other >

Explanation: First, simplify the information given. The first three minutes, at 42 cents per minute, costs a total of \$1.26. The total cost of the call can be given as:

1.26 + 0.18(a)

where a is the number of additional minutes. Alternatively, since the number of "additional minutes" is 3 minutes less than the length of the call, we can use this:

1.26 + 0.18(m - 3)

where m is the total number of minutes of the call.

Statement (1) is sufficient. We know that the first three minutes cost \$1.26. If that's 36 cents less than the cost of the remainder of the call, the remainder of the call cost:

\$1.26 + \$0.36 = \$1.62.

We can divide that number by 18 cents to determine the number of additional minutes.

Statement (2) is also sufficient. Using the formula we derived from the question:

1.26 + 0.18(m - 3) = 2.88

Here, we have only one variable in a linear equation, so we have enough information to solve. At this point you can conclude that (D) is the correct answer. To solve it for good measure:

1.26 + 0.18(m - 3) = 2.88

0.18(m - 3) = 1.62

m - 3 = 9

m = 12

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