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DavidG@VeritasPrep Legendary Member
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Tue Aug 15, 2017 10:00 am
Come to think of it, we don't have to think about this one very hard.

Together:
Scenario 1: She gets the distance and the rate exactly right - YES, she's within 5. hours of the actual time.

Scenario 2: actual rate = 0; estimated rate = 10. Clearly, NO, she's not within .5 hours of the actual time, no matter the distance. She's not moving.

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NadineKh Newbie | Next Rank: 10 Posts
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Tue Aug 15, 2017 10:19 am
Thank you very much!!

It also helped me interpreting it more intuitively: since we have delta (differences between estimation and reality) in both numerator and denominator, we can't have an idea of the % deviation without knowing the value of the ratio.

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Brent@GMATPrepNow GMAT Instructor
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Wed Nov 15, 2017 4:35 pm
In planning for a car trip, Joan estimated both the distance of the trip, in miles, and her average speed, in miles per hour. She accurately divided her estimated distance by her estimated average speed to obtain an estimate for the time, in hours, that the trip would take. Was her estimate within 0.5 hour of the actual time that the trip took?
(1) Joanâ€™s estimate for the distance was within 5 miles of the actual distance.
(2) Joanâ€™s estimate for her average speed was within 10 miles per hour of her actual average speed.

Target question: Was Joan's ESTIMATE within 0.5 hour of the ACTUAL TIME that the trip took?

Statement 1: Joanâ€™s ESTIMATE for the distance was within 5 miles of the ACTUAL distance.
Travel time = distance/speed

Statement 1 provides information regarding the accuracy of Joan's estimation of the travel distance, BUT it does not provide any information regarding her accuracy in estimating her speed.
As such, statement 1 is NOT SUFFICIENT

Statement 2: Joanâ€™s estimate for her average speed was within 10 miles per hour of her actual average speed.
Statement 2 provides information regarding the accuracy of Joan's estimation of her average speed, BUT it does not provide any information regarding her accuracy in estimating the travel distance.
As such, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Let's test some numbers.
There are several possible scenarios that satisfy BOTH statements. Here are two:
Case a: Joan's estimates were PERFECTLY accurate. In this case, her ACTUAL travel time was definitely WITHIN 0.5 hours of her ESTIMATED travel.

Case b: Joan's ESTIMATED distance and average speed were 8 miles and 8 miles per hour respectively, and the ACTUAL distance and average speed were 5 miles and 1 mile per hour respectively. So, Joan's ESTIMATED travel time = 8/8 = 1 hour, and her ACTUAL travel time = 5/1 = 5 hours. In this case, Joan's ACTUAL travel time was NOT WITHIN 0.5 hours of her ESTIMATED travel.

Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

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