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How many hours did it take Helen to drive to her parents

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Intern
Joined: 13 Mar 2006
Posts: 9
Location: Vancouver
How many hours did it take Helen to drive to her parents [#permalink]

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03 May 2006, 19:14
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How many hours did it take Helen to drive to her parents house?

1) her average speed was 72km/h
2) if her average speed was +8km/h it would have taken her 1 hr less

OA is C
Director
Joined: 16 Aug 2005
Posts: 938
Location: France

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03 May 2006, 19:59
speed = distance/time

Let time taken be x hours

1. Not sufficient

2. Not sufficient

Together:
Distance travelled at 72km/h driving for x hours = 72x km

Distance travelled at 80km/h for (x-1) hours = 80(x-1) km

Since distance is same, 72x = 80x - 80
=> 8x = 80 => x=10 hours

VP
Joined: 06 Jun 2004
Posts: 1053
Location: CA

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03 May 2006, 21:38
Agree with gmatmba's approach

Also, please do not post the OA in advance so ppl could have a chance to work on it. Tks
_________________

Don't be afraid to take a flying leap of faith.. If you risk nothing, than you gain nothing...

GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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03 May 2006, 22:04
St1:
We do not know the distance. Insufficient.

St2:
Average speed = s
Distance = d
Time = d/s

If average speed = (s+8)
Time = d/(s+8) = d/s - 1

d/(s+8) = (d-s)/s
ds = (s+8)(d-s)
ds = ds - s^2 + 8d - 8s
s^2 + 8s - 8d = 0

Can't solve as we do not knwo real values of s and d.
Insufficient.

Using St1 and St2:
s^2 + 8s - 8d = 0
75^2 + 8(75) = 8d

Can calculate d and therefore the time taken. Sufficient.

Ans C
03 May 2006, 22:04
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