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A car travels from point A to point B. The average speed of

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A car travels from point A to point B. The average speed of [#permalink] New post 26 Jul 2010, 22:46
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A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?

A. 30
B. 45
C. 60
D. 75
E. 90
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Re: distance problem [#permalink] New post 26 Jul 2010, 23:08
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aiming4mba wrote:
A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?


Let the average speed for the second half of the trip be x miles per hour.

Pick some smart number for the distance from A to B: let the distance be 180 miles, so half of the distance will be 90 miles.

Average \ speed=\frac{total \ distance}{total \ time} --> 60=\frac{180}{total \ time};

Total time equals time for the first half plus time for the second half --> \frac{90}{90}+\frac{90}{x}=\frac{x+90}{x};

So 60=\frac{180}{total \ time} --> 60=\frac{180x}{x+90} --> 60x+60*90=180x --> 120x=60*90 --> x=45.

Answer: 45.
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A car travels from point A to point B. [#permalink] New post 16 Nov 2012, 13:58
A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?

A. 30
B. 45
C. 60
D. 75
E. It cannot be determined

Could someone explain it, please? It would be ideal if someone explain it using the MGMAT approach, the VT = D chart.
Thanks!

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Re: A car travels from point A to point B. [#permalink] New post 16 Nov 2012, 14:04
danzig wrote:
A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?

A. 30
B. 45
C. 60
D. 75
E. It cannot be determined

Could someone explain it, please? It would be ideal if someone explain it using the MGMAT approach, the VT = D chart.
Thanks!

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Merging similar topics. Pleas refer to the solution above and ask if anything remains unclear.
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DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!


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Re: A car travels from point A to point B. The average speed of [#permalink] New post 16 Nov 2012, 15:15
Thanks Bunuel!

An additional question: What is the logic behind picking numbers in this problem? , How do these numbers lead us to the answer?
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Re: A car travels from point A to point B. The average speed of [#permalink] New post 18 Nov 2012, 14:34
danzig wrote:
Thanks Bunuel!

An additional question: What is the logic behind picking numbers in this problem? , How do these numbers lead us to the answer?



180 is the least common multiple of 90 mi/hr and 60 mi/hr. Picking 180 makes calculations easy when you know d = rt
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Re: A car travels from point A to point B. The average speed of [#permalink] New post 27 Nov 2012, 03:28
how about weighted average for this method? the number between 90 and 60 is 30. So it should be slightly higher than 30 and less than 60. So the ans is 45
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Re: A car travels from point A to point B. The average speed of   [#permalink] 27 Nov 2012, 03:28
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