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Official Guide Explanation:Problem Solving #100

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: 100
Page: 166
Difficulty: 4 (Moderately Easy)
Category 1: Algebra > Linear Equations-One Unk >

Explanation: Plugging in 290 for T is simple enough; the challenge in this question is efficiently handling the calculation. Here's a useful approach:

290 = (5/9)(K - 32)

290((9/5)) = K - 32

Now, divide 290 by 5. That can be done in your head: just double 290 (result: 580) and divide by 10 (result: 58).

58(9) + 32 = K

The product of 58 and 9 is trickier, but still doable with mental math tricks. 58(9) is really 58(10 - 1), so multiply 58 by 10 (result: 580) and subtract 58 (result: 522). Add 32, and you have your answer: 522 + 32 = 554. It may look like a lot of steps, but it's faster and more error - proof than long division and pencil - and - paper multiplication.

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