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## Official Guide Explanation:Data Sufficiency #104

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: 104
Page: 282
Difficulty: 5 (Moderate)
Category 1: Word Problems > Rate Problems > Speed

Explanation: This question tests whether you can distinguish between "average rate" and rate at a given moment. If you take one hour to drive a 50 mile route, your average speed is 50 miles per hour, but that doesn't mean you were driving 50 miles per hour the entire time--you might have driven 70 miles per hour part of the time, 45 miles per hour part of the time, and 25 miles per hour part of the time. It's much more difficult to answer "what's the rate at a given moment" questions than "average rate" questions.

Statement (1) is insufficient. You can calculate average rate, but there's no information about the specific moment when the train had completed half of its trip.

Statement (2) is also insufficient. Again, this gives you the average rate, but no more.

Taken together, the statements are still insufficient. In fact, the two statements give you the same information. Each one tells you the average rate, so neither helps you find the rate at any given moment. Choice (E) is correct.

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