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While driving on the expressway, did Robin ever exceed

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While driving on the expressway, did Robin ever exceed [#permalink]  21 Feb 2011, 05:56
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Question Stats:

83% (01:19) correct 17% (00:22) wrong based on 36 sessions
93) While driving on the expressway, did Robin ever exceed the 55- mile-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
[Reveal] Spoiler: OA

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Re: Robin ever exceed the 55mile/hr [#permalink]  21 Feb 2011, 06:02
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: Robin ever exceed the 55mile/hr [#permalink]  21 Feb 2011, 06:04
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: Robin ever exceed the 55mile/hr   [#permalink] 21 Feb 2011, 06:04
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