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While driving on the expressway, did Robin ever exceed the 55-miles-pe

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While driving on the expressway, did Robin ever exceed the 55-miles-pe  [#permalink]

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New post 21 Feb 2011, 06:56
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A
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E

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Question Stats:

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While driving on the expressway, did Robin ever exceed the 55-miles-per-hour speed limit?

(1) Robin drove 100 miles on the expressway.
(2) Robin drove for 2 hours on the expressway.

1) to calculate speed to cover 100 miles time is required ----NS
2) in 2 hours how many miles Robin covered we don't know.---NS

combining 1&2 options

speed = 100/2=50 answer is no he didn't exceed speed

c

Which is wrong

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Re: While driving on the expressway, did Robin ever exceed the 55-miles-pe  [#permalink]

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New post 21 Feb 2011, 07:02
1
100/2=50 m/h is his average speed.

What if he drove @ 60m/h in first hour and 40 miles/h in the second;

Total distance: 100miles
time: 2 hours
But he sped up to 60m/h > 55m/h

What if he drove @ constant rate of 50m/h in first hour and constant rate of 50 miles/h in the second;

Total distance: 100miles
time: 2 hours
But he didn't speed beyond 55m/h

Ans: "E"
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Re: While driving on the expressway, did Robin ever exceed the 55-miles-pe  [#permalink]

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New post 21 Feb 2011, 07:04
That's because when you divide the distance by time, you get the average speed...

However, the question is, did Robin exceed the 55 mph speed limit?

So the average speed has no significance here :) We require the peak speed!

So using statement 1 and 2:
Robin drove for 2 hours and covered a distance of 100 miles:
Case 1
If Robin drove at 60 mph for the first hour, distance = 60 miles
If Robin drove at 40 mph for the next hour, distance = 40 miles
Total Distance = 100
Total time = 2 hrs
Did she exceed the speed limit? YES!
Case 2:
If Robin drove at 50 mph for 2 hours, distance = 100 miles
Total Distance = 100
Total Time = 2 hrs
Did she Exceed the speed limit? NO!

Thus, insufficient!
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Re: While driving on the expressway, did Robin ever exceed the 55-miles-pe  [#permalink]

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New post 22 Dec 2017, 12:03
Top Contributor
GMATD11 wrote:
While driving on the expressway, did Robin ever exceed the 55-miles-per-hour speed limit?

(1) Robin drove 100 miles on the expressway.
(2) Robin drove for 2 hours on the expressway.


NOTE: There's nothing here to suggest that Robin's speed was constant. Sure, she MIGHT have traveled at a constant speed, or her speed may have varied throughout the trip.

Target question: Did Robin's speed ever exceed 55 miles per hour?

Statement 1: Robin drove 100 miles
No information about Robin's travel TIME so statement 1 is NOT SUFFICIENT
Consider these two possible scenarios that yield conflicting answers to the target question:
Case a: driving at a constant speed, Robin spent 1 hour driving 100 miles, in which case her speed was 100 miles per hour. So, Robin's speed DID exceed 55 miles per hour
Case b: driving at a constant speed, Robin spent 10 hours driving 100 miles, in which case her speed was 10 miles per hour. So, Robin's speed did NOT exceed 55 miles per hour
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: Robin drove for 2 hours
No information about the DISTANCE Robin traveled. So, statement 2 is NOT SUFFICIENT
Consider these two possible scenarios that yield conflicting answers to the target question:
Case a: driving at a constant speed, Robin spent 2 hours driving 1000 miles, in which case her speed was 500 miles per hour. So, Robin's speed DID exceed 55 miles per hour
Case b: driving at a constant speed, Robin spent 2 hours driving 2 miles, in which case her speed was 1 mile per hour. So, Robin's speed did NOT exceed 55 miles per hour
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Consider these two possible scenarios that yield conflicting answers to the target question:
Case a: driving at a constant speed, Robin spent 2 hours driving 100 miles, in which case her speed was 50 miles per hour. So, Robin's speed did NOT exceed 55 miles per hour
Case b: Robin drove 99 miles (at a constant speed) for 1 hour, and then spent 1 hour driving the remaining 1 mile. So, Robin's speed DID exceed 55 miles per hour (during the 1st hour)
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer: E

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Brent
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Re: While driving on the expressway, did Robin ever exceed the 55-miles-pe &nbs [#permalink] 22 Dec 2017, 12:03
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