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Official Guide Explanation:Problem Solving #61

Background

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.

Question: 61
Page: 69
Difficulty: 4 (Moderately Easy)
Category 1: Word Problems > Other >
Category 2: Algebra > Linear Equations-One Unk >

Explanation: Initially the tank was at (3/4) capacity, meaning that if capacity is c, the amount of water can be given by (3/4)c. After 4,000 gallons were added, the amount of water was (4/5)c. We can express this in a single equation:

(3/4)c + 4,000 = (4/5)c

We're looking for capacity, so solve for c:

4,000 = (4/5)c - (3/4)c

4,000 = (16/20)c - (15/20)c

4,000 = (1/20)c

c = 4,000(20) = 80,000, choice (E).

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