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A car traveling at a certain constant speed takes 2 seconds

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VP
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A car traveling at a certain constant speed takes 2 seconds [#permalink] New post 19 Jan 2008, 06:11
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?

A. 71.5
B. 72
C. 72.5
D. 73
E. 73.5
Director
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Re: SPEED [#permalink] New post 19 Jan 2008, 06:25
Hi,
The car travels 1 km at 75 kmh for 48 seconds (75000/3600=250/12 mps) 1000/(250/12)

Then for 50 sec the car travels 1 km. The speed of the car is 1000/50 = 20 mps or 72 kmph
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Re: SPEED [#permalink] New post 19 Jan 2008, 11:33
marcodonzelli wrote:
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?

A. 71.5
B. 72
C. 72.5
D. 73
E. 73.5


R 75 X
T 1/75 hr
D 1 1

2s/1*1min/60sec*1hr/60min

2/3600 --> 1/1800

1/75+1/1800 = Total time 5*5*3, 8*5*5*3*3 --> 24/1800 + 1/1800 --> 25/1800

1/25/1800 --> 1800/25 --> 72

B
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Re: SPEED [#permalink] New post 19 Jan 2008, 11:39
marcodonzelli wrote:
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?

A. 71.5
B. 72
C. 72.5
D. 73
E. 73.5


When the car travels at 75 kmph,
In 3600 sec --> 75 kms
? sec ---> 1 km

So for 1 km, 3600/75 = 48 sec.

From the problem, it takes 2 sec more than 48= 50 sec.

So it follows,
1 km -- > 50 sec
? km --> 3600 sec

So speed will be 3600 /50 = 72 kmph
Answer is B
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Re: SPEED [#permalink] New post 21 Jan 2008, 14:58
different approach:

75 km/h ==> 75km in 60 mins
the slower speed takes 2secs per km longer, so it would travel 75km in 62.5 mins

so its speed is 75 * 60/62.5

==> 75 * 120/125

easy to simplify now ==> 3*120/5

==> 3*24 = 72 km/h


p.s so nice to get a question in metrics instead of miles (not that it matters)
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Re: SPEED [#permalink] New post 21 Jan 2008, 15:19
75km an hour = 75,000 meters in 60 minutes

This means 1,250 meters per minute at 75kmh.

We want to know how long it takes to go 1,000 meters, so divide by 5 and multiply again by 4. 60/5=12*4=48 seconds.

Add 2 seconds to see 50 seconds to go 1,000 meters = divide by 5 and multiply by 6. 1,200 meters per minute. Multiply by 60 = 72,000 meters per hour.

72kmh

Answer B and you can do it all in your head 8-)
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Re: SPEED [#permalink] New post 22 Jan 2008, 01:53
eschn3am wrote:
75km an hour = 75,000 meters in 60 minutes

This means 1,250 meters per minute at 75kmh.

We want to know how long it takes to go 1,000 meters, so divide by 5 and multiply again by 4. 60/5=12*4=48 seconds.

Add 2 seconds to see 50 seconds to go 1,000 meters = divide by 5 and multiply by 6. 1,200 meters per minute. Multiply by 60 = 72,000 meters per hour.

72kmh

Answer B and you can do it all in your head 8-)


Your approach helpful, but I can still not get it. Why divide by 5, multiply by 4 and why divide by 5 and multiple by 6. I am a little confused.
Re: SPEED   [#permalink] 22 Jan 2008, 01:53
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