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How many hours did it take Helen to drive from her house to

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How many hours did it take Helen to drive from her house to [#permalink]

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New post 23 Jul 2008, 22:50
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Question Stats:

73% (00:53) correct 27% (00:33) wrong based on 119 sessions

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How many hours did it take Helen to drive from her house to her parent's house?

(1) Helen's average speed on this trip was 72 kilometers per hour
(2) If Helen's average speed on this trip had been 8 kilometers per hour greater , it would have taken her 1 hour less.

OPEN DISCUSSION OF THIS QUESTION IS HERE: how-many-hours-did-it-take-helen-to-drive-from-her-house-to-91891.html
[Reveal] Spoiler: OA

Last edited by Bunuel on 30 Aug 2013, 04:08, edited 1 time in total.
Moved to DS forum and added the OA.

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Re: Complex rate problem [#permalink]

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New post 23 Jul 2008, 22:58
Statement 1: Alone is insufficient as distance is unavailable.

Statement 2: Alone is insufficient as neither speed nor distance is available.

Combining Both: From Statement 1 we know that 72 km/hr is the avg. speed. Let the time taken by helen be X.

Thereforhome.e distance travelled by Helen is 72*X

From statement 2, we know that if the speed had been 8 km/hr more than the original it would take 1 hr less.

=> 72X/80 = X-1

Therefore we can solve for X, which is the time take by Helen.
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Re: Complex rate problem [#permalink]

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New post 24 Jul 2008, 03:55
(1) cannot say anything

Let x = time taken in 72 km / hr case

since distance = speed * time

72x = (72 + 8) (x -1)
72x = 80(x - 1)
8x = 80
x = 10

so (2) sufficient

so B

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Re: Complex rate problem [#permalink]

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New post 24 Jul 2008, 04:05
djdela wrote:
How many hours did it take Helen to drive from her house to her parent's house?

(1) Helen's average speed on this trip was 72 kilometers per hour
(2) If Helen's average speed on this trip had been 8 kilometers per hour greater , it would have taken her 1 hour less.


i suck at Q..what can i say..

D/V=t

D/(v+8)=t+1

Vt=(t+1)(v+8)
vt+8t+v=vt

8t+v=0...we dont know V and and t..

1) we are told v=72

1+2) 8t=72 t=9

i get C..

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Re: Complex rate problem [#permalink]

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New post 24 Jul 2008, 04:10
sset009 wrote:
(1) cannot say anything

Let x = time taken in 72 km / hr case

since distance = speed * time

72x = (72 + 8) (x -1)
72x = 80(x - 1)
8x = 80
x = 10

so (2) sufficient

so B

oops used, info from 1
should be C

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Re: Complex rate problem [#permalink]

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New post 24 Jul 2008, 08:45
How many hours did it take Helen to drive from her house to her parent's house?

(1) Helen's average speed on this trip was 72 kilometers per hour
(2) If Helen's average speed on this trip had been 8 kilometers per hour greater , it would have taken her 1 hour less.


(1) alone is NOT sufficient. it just gives speed, no information on time and distance.

(2) alone is NOT sufficient. We can derive a relationship: (let x be speed, t = time)
x*t = (x+8)*(t-1)

(1) +(2), then 72t = 80(t-1)
Sufficient

Ans is C

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Re: How many hours did it take Helen to drive from her house to [#permalink]

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New post 29 Aug 2013, 21:01
D/72-D/80=1
solve for D. we get D=720
COMBINE . answer is C.

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Re: How many hours did it take Helen to drive from her house to [#permalink]

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New post 30 Aug 2013, 04:08

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Re: How many hours did it take Helen to drive from her house to   [#permalink] 30 Aug 2013, 04:08
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