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by sunman » Wed Jun 27, 2012 8:06 am
A train traveling at a certain constant speed takes 30 seconds longer to travel 2 kilometers than it would take to travel 4 kilometers at 120 kilometers per hour. At what speed, in kilometers per hour, is the train traveling?

A. 40
B. 48
C. 80
D. 96
E. 120

I selected B, but apparently the answer was C? I don't see it.

Work below:

120 kph for 4 km =

120 kph / 60 minutes = 2 kpm (km per minute)

@ 4 km = 2 minutes

The train takes 30 seconds longer to travel 2km than it does to travel 4km, so:

Time = 2:30
Distance = 2km

2km/2.5 minutes = 0.8 minutes per km

0.8 x 60 minutes = 48 km.

Why is the answer 80? Please help.
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by Brent@GMATPrepNow » Wed Jun 27, 2012 8:21 am
sunman wrote:A train traveling at a certain constant speed takes 30 seconds longer to travel 2 kilometers than it would take to travel 4 kilometers at 120 kilometers per hour. At what speed, in kilometers per hour, is the train traveling?

A. 40
B. 48
C. 80
D. 96
E. 120

I selected B, but apparently the answer was C? I don't see it.

Work below:

120 kph for 4 km =

120 kph / 60 minutes = 2 kpm (km per minute)

@ 4 km = 2 minutes

The train takes 30 seconds longer to travel 2km than it does to travel 4km, so:

Time = 2:30
Distance = 2km

2km/2.5 minutes = 0.8 minutes per km (I think you mean km per minute)

0.8 x 60 minutes = 48 km.

Why is the answer 80? Please help.
Your solution looks good to me (with a slight correction above in blue)

Cheers,
Brent
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by sunman » Wed Jun 27, 2012 8:45 am
Brent@GMATPrepNow wrote:
sunman wrote:A train traveling at a certain constant speed takes 30 seconds longer to travel 2 kilometers than it would take to travel 4 kilometers at 120 kilometers per hour. At what speed, in kilometers per hour, is the train traveling?

A. 40
B. 48
C. 80
D. 96
E. 120

I selected B, but apparently the answer was C? I don't see it.

Work below:

120 kph for 4 km =

120 kph / 60 minutes = 2 kpm (km per minute)

@ 4 km = 2 minutes

The train takes 30 seconds longer to travel 2km than it does to travel 4km, so:

Time = 2:30
Distance = 2km

2km/2.5 minutes = 0.8 minutes per km (I think you mean km per minute)

0.8 x 60 minutes = 48 km.

Why is the answer 80? Please help.
Your solution looks good to me (with a slight correction above in blue)

Cheers,
Brent
Hmmm...error in GMATClub test question then.

Thanks Brent!
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by Brent@GMATPrepNow » Wed Jun 27, 2012 8:47 am
sunman wrote: Hmmm...error in GMATClub test question then.
Unless I'm somehow missing something . . .

Cheers,
Brent
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by sunman » Wed Jun 27, 2012 8:58 am
I doubt you are. Mistakes happen, even for people who make questions.
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by GMATGuruNY » Wed Jun 27, 2012 10:36 am
sunman wrote:A train traveling at a certain constant speed takes 30 seconds longer to travel 2 kilometers than it would take to travel 4 kilometers at 120 kilometers per hour. At what speed, in kilometers per hour, is the train traveling?

A. 40
B. 48
C. 80
D. 96
E. 120

I selected B, but apparently the answer was C? I don't see it.

Work below:

120 kph for 4 km =

120 kph / 60 minutes = 2 kpm (km per minute)

@ 4 km = 2 minutes

The train takes 30 seconds longer to travel 2km than it does to travel 4km, so:

Time = 2:30
Distance = 2km

2km/2.5 minutes = 0.8 minutes per km

0.8 x 60 minutes = 48 km.

Why is the answer 80? Please help.
Answer choice C would be correct if the times were reversed -- if it took 30 seconds longer to travel 4 kilometers at 120kph than to travel 2 kilometers at the actual speed.

Time to travel 4 kilometers at 120kph = d/r = 4/120 of an hour.
Time to travel 2 kilometers at 80kph = d/r = 2/80 = 1/40 of an hour.
Time difference = 4/120 - 1/40 = 1/120 of an hour, which is equal to 30 seconds.
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by sunman » Wed Jun 27, 2012 9:01 pm
Thanks. That was a copy and paste, so seems like GMATClub messed a question up.
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