A car traveling at a certain constant speed...

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

A car traveling at a certain constant speed...

by BTGmoderatorLU » Tue Feb 27, 2018 10:53 am

A car traveling at a certain constant speed takes 5 seconds longer to travel 1 km than it would take to travel 1 km at 80 km/h. At what speed, in km/h, is the car traveling?

A. 70
B. 72
C. 74
D. 75
E. 78

The OA is B.

I know that 1 kilometer at 80 km/h is covered in a time = 1/80 hours, in seconds will be 3600/80 = 45 seconds. I stuck here.

I'm confused by this PS question. Experts, any suggestion about how can I solve it? Thanks in advance.

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Source: Beat The GMAT — Problem Solving | Tue Feb 27, 2018 10:53 am

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by [email protected] » Tue Feb 27, 2018 4:47 pm

Hi LUANDATO,

We're told that a car traveling at a certain constant speed takes 5 seconds longer to travel 1 km than it would take to travel 1 km at 80 km/hour. We're asked at what speed, in km/hour, the car is traveling.

Your 'first step' - to convert km/hour into seconds/km is perfect. Since there are 3600 seconds per hour, it would take 3600/80 = 45 seconds to travel 1 km when traveling 80km/hour.

We're told that it takes 5 seconds LONGER to travel that same distance, so we now have to add those 5 seconds and 'work backwards' to find the speed...

(45+5) = 50 seconds to travel 1 km

Since there are 3600 seconds per hour, then we would end up traveling 3600/50 = 72 km in one hour. Thus, the speed is 72 km/hour.

Final Answer: B

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Rich

Contact Rich at [email protected]

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pm; Location: New York, NY; Thanked: 13060 times; Followed by:1906 members; GMAT Score:790

by GMATGuruNY » Wed Feb 28, 2018 3:45 am

LUANDATO wrote:A car traveling at a certain constant speed takes 5 seconds longer to travel 1 km than it would take to travel 1 km at 80 km/h. At what speed, in km/h, is the car traveling?

A. 70
B. 72
C. 74
D. 75
E. 78

80 kilometers per hour = 80 kilometers in 3600 seconds.
A car takes 5 seconds longer.
Since 1 kilometer takes 5 seconds longer, 80 kilometers takes 400 seconds longer, implying a rate of 80 kilometers in 4000 seconds:
(80 kilometers)/(4000 seconds) * (3600 seconds)/(1 hour) = 72 kph.

The correct answer is B.

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by Scott@TargetTestPrep » Thu Mar 01, 2018 5:39 pm

LUANDATO wrote:A car traveling at a certain constant speed takes 5 seconds longer to travel 1 km than it would take to travel 1 km at 80 km/h. At what speed, in km/h, is the car traveling?

A. 70
B. 72
C. 74
D. 75
E. 78

If a car is traveling at 80 km per hour, then the number of seconds (x) it takes to travel 1 km can be calculated using the following proportion (note that 1 hour = 3,600 seconds):

80/3600 = 1/x

80x = 3600

x = 45 sec

Thus, it takes 45 seconds to travel 1 km if the speed is 80 km/h. Since the car actually takes 5 more seconds (i.e., 50 seconds) to travel 1 km, its speed (r) can be calculated using the following proportion:

r/3600 = 1/50

50r = 3600

r = 72 km/h

Answer: B

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