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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] New post 26 Mar 2010, 16:00
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C
D
E

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Question Stats:

79% (01:39) correct 21% (00:38) wrong based on 119 sessions
How many hours did it take Helen to drive from her house to her parents 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.

Thanks in advance!
[Reveal] Spoiler: OA
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Re: GMAT Prep Q [#permalink] New post 26 Mar 2010, 16:10
Expert's post
changhiskhan wrote:
How many hours did it take Helen to drive from her house to her parents 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.

Thanks in advance!



d=rt, where d is the distance covered, r rate and t time. Question: t=?
(1) r=72. Clearly not sufficient.
(2) (r+8)(t-1)=d. Clearly not sufficient.

(1)+(2) r=72 and (r+8)(t-1)=d --> 80(t-1)=d --> d=80(t-1)=rt=72t --> 8t=80 --> t=10. Sufficient.

Answer: C.
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Re: GMAT Prep Q [#permalink] New post 27 Mar 2010, 07:52
changhiskhan wrote:
How many hours did it take Helen to drive from her house to her parents 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.

Thanks in advance!


1: Not sufficient, easy to spot
2: distance is d
av speed: a then time is t
av speed: a-1 then time is t-1
d/a = d/a+8 + 1 can't solve alone insufficient.

Combining, d/72 = d/80 + 1 solving d = 80 then t can be calculated hence C.
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Driving to Helen's house - DS problem [#permalink] New post 27 Apr 2010, 04:35
How many hours did it take Helen to drive from her house to her parents's house?


(1) Helen's average speed on this trip was 72 km/hr.
(2) If Helen's average speed on this trip has been 8 km/hr greater, it would have taken her 1 hour less.

[Reveal] Spoiler:
C


Source: GMAT Prep

Can someone explain this clearly.
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Re: Driving to Helen's house - DS problem [#permalink] New post 27 Apr 2010, 04:57
zz0vlb wrote:
How many hours did it take Helen to drive from her house to her parents's house?


(1) Helen's average speed on this trip was 72 km/hr.
(2) If Helen's average speed on this trip has been 8 km/hr greater, it would have taken her 1 hour less.

[Reveal] Spoiler:
C


Source: GMAT Prep

Can someone explain this clearly.


Hi,

Its C.

You need to calculate the distance, let that be D.

Now from 1 you just get the speed. To know D you need time and speed both. so 1 is insufficient

Using 2 you just get the difference in speed and the time, again no concrete values. so 2 is insufficient.

However if you combine 1 and 2 you get 2 speeds and the difference in time.

Here is how the solution would be

Earlier speed = 72.
time taken at 72 km/hr = t1
t1 = D/72


Later speed = 72+8 = 80 km/hr
Time taken at 80 km/hr = t2
t2 = D/80

Now t1-t2 = 1

D/72 -D/80 = 1

Solve this and you get D = 720 km.
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Re: GMAT Prep Q [#permalink] New post 15 Mar 2011, 22:39
changhiskhan wrote:
How many hours did it take Helen to drive from her house to her parents 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.

Thanks in advance!


C after 3:40 minutes :cry:
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Re: GMAT Prep Q [#permalink] New post 16 Mar 2011, 18:37
Let distance from her house to her parents house = D, average speed v, and time taken be t

(1) is insufficient, we only have t = D/72


From (2), D/(v+8) = D/v - 1

So this is not sufficient either, but if we combine (1) and (2), v = 72 and hence we can find out both. So the answer is C.
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Re: GMAT Prep Q [#permalink] New post 18 Mar 2011, 01:10
basically, we are provided with two equations in 1statement 1 and 2.
having two equations it is possible to solve equality with two uknowns.

hence it is C.

30 seconds.
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Re: How many hours did it take Helen to drive from her house to [#permalink] New post 26 Nov 2012, 06:38
Could you also solve this problem by thinking of it as a ratio? One of MGMAT's strategies for DS ratio questions is that if a DS question asks for the concrete value of one element of a ratio, you will need both the concrete value of another element and the relative value of 2 elements of the ratio. When you combine the two statements, you get a concrete value from statement 1 and two relative values from statement 2. Is that a logical way to solve this problem, once you already know that the two statements individually are insufficient?
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Re: How many hours did it take Helen to drive from her house to [#permalink] New post 01 Aug 2013, 10:52
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How many hours did it take Helen to drive from her house to her parents house?

1) Helen’s average speed on this trip was 72 kilometers per hour.
We know nothing about distance. If the distance was 72km then it would have taken one hour. If the distance was 720km then it would have taken 10 hours, etc. All we know is r=72
INSUFFICIENT

2) If Helen’s average speed on this trip had been 8 kilometers per hour greater, it would have taken her 1 hour less.
Time=distance/speed
T-1 = d/(s+8)
This does not provide enough information to solve for the question.
INSUFFICIENT

1+2) average speed (r) = 72, t-1 = d/(s+8)
t-1 = d/(72+8)
t-1 = d/80
This is where it gets tricky. At first I said that this isn't enough information to solve so I put down (E). However, d=r*t. If I plug in for d then I can cancel out t leaving r.
t-1 = r*t/(72+8)
t-1 = 72*t/(80)
80t-80 = 72t
8t = 80
t=10

A couple things to remember:
- If in doubt and unable to solve, try and substitute in a formula (like=r*t) to cancel out variables.
- remember to plug in any applicable numbers for all variables. For example, after plugging in r*t for d, I forgot to plug in 72 for r*t.

SUFFICIENT

(C)
Re: How many hours did it take Helen to drive from her house to   [#permalink] 01 Aug 2013, 10:52
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