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by beater » Mon Sep 22, 2008 12:25 pm
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

Could you please explan using RTD chart. Thansk a lot!
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by k_abhi19 » Mon Sep 22, 2008 3:27 pm
The answer is B... a simpler approach can be to think it like this...

when travelling at 75kmph.. the car travels 1 km in 48 secs (as 1/75 hrs or 3600/75=48 secs) and thus we need to find out the speed when car travels 1 km in 50 (48 + 2) secs.... now 50 secs is nothing but 1/72 hr (3600 secs = 1hrs i.e 50 secs = 1/72 hrs).. thus speed will be Dist/time i.e 1/(1/72) kmph.. or 72kmph...
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by khudania » Tue Sep 23, 2008 2:37 am
Right on - 72
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by beater » Tue Sep 23, 2008 9:45 am
Anyone with a different approach. I think the key to such problems is to make sure the units (hr/sec/km/miles)are consistent. Thanks!
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by gixxer1000 » Wed Sep 24, 2008 11:31 am
I did it the tradition way but it will probably take longer.

R*T=D, So:

75km/hr * T = 1km

T = 1/75hr

Then add two seconds, but first convert it two hrs

2sec * 1min/60sec * 1hr/60min = 1/1800hr

1/75hr + 1/1800hr = 1875/135000

Simplify

1/72hr

Now use this time to solve for the rate:

R * 1/72hr = 1km

R = 72km/hr
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by allechat » Fri Oct 10, 2008 8:01 am
Or, if you convert to seconds, like done above, you can then plug in the given answers for 48 seconds and 50 seconds and see that 72 works. (75*48=3600) , (72*50=3600)
You still have to solve for 48 and 50 as the time.
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by rahul.s » Wed Jan 13, 2010 11:27 pm
75 kms 3600 seconds
1 km ? seconds

3600/75 = 48 seconds to cover a distance of 1 km.

Now, we have to find the speed of the car per hour that takes 50 seconds to cover a distance of 1 km

50 seconds 1 km
3600 seconds ? kms

3600/50 = 72

So, it's B :)
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by mcdesty » Thu Jul 24, 2014 7:30 pm
My scratch paper looked like this when I tackled this one.
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by [email protected] » Thu Jul 24, 2014 9:00 pm
Hi All,

It looks like most of the users in this post are long gone (2008 to 2010), but for anyone who might be interested, there are other ways to "do the math" in this prompt:

75 kilometers per hour is the same as 75 kilometers per 60 minutes....

75/60 = 5/4 = 5 kilometers every 4 minutes

This means that every 1 kilometer takes 4/5 minutes = 48 seconds

We're then asked to add 2 seconds to this and calculate the speed.

48 + 2 = 50 seconds

To find the speed, we can use a ratio (50 seconds = 5/6 minutes):

5/6 = 60 min/X kph

Cross multiply the ratio:

5X = 360

X = 72 kph

Final Answer: B

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by GMATGuruNY » Thu Jul 24, 2014 9:23 pm
beater 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
If 1 kilometer requires 2 more seconds, then 75 kilometers requires 150 more seconds.
Implication:
The rate of the car =

= (75 kilometers)/(1 hour + 150 seconds)

= (75 kilometers)/(3750 seconds)

= 1 kilometer/50 seconds

= 2 kilometers/100 seconds

= 72 kilometers/3600 seconds

= 72 kilometers per hour.

The correct answer is B.
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by abhasjha » Thu Jul 24, 2014 9:59 pm
few things that come handy before you proceed to solve the problem :

(1) zkm/hr = z(5/18) m/s ... ( use conversion 1 km= 1000m ;an hour = 3600 seconds)
(2) zmeter/second = z(18/5) km/hour .

now on to the question


75 kilometers per hour= 75(5/18)m/second
distance = 1 km = 1000m

thefore time taken in this case = distance / speed. 1000/(75x5)/18 = 48 seconds

A car traveling at a certain constant speed takes 2 seconds longer . therefore actual time taken = 48+2= 50 seconds .

distance = 1 kilometer .

so speed = (1000meter) /50 . = 20meter/s

converting 20m/s imto km/hr = 20x18/5 = 72km/hr.
so answer B.
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by GMATinsight » Fri Jul 25, 2014 3:13 am
beater 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

Could you please explan using RTD chart. Thansk a lot!
If Speed of Car = S

TIME = DISTANCE / SPEED

TIME to travel 1 KM at 75 KMPH = 1/75 Hour = 3600/75 Sec [Because 1 Hour = 3600 Secs]
TIME to travel 1 KM at S KMPH = 1/S Hour = 3600/S Sec

Given, (3600/S) - (3600/75) = 2 Seconds
i.e. (3600/S) - 48 = 2 Seconds
i.e. (3600/S) = 50 Seconds
i.e. S = 72 KM/HOUR

Answer: Option B
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by Jeff@TargetTestPrep » Wed Dec 13, 2017 6:19 am
beater 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
We can let r = the speed of the car in km per hour; thus it would take the car 1/r hour to travel 1 km. If the car traveled at 75 km per hour, then it would take the car 1/75 hour to travel 1 km.

We are given that the car would be 2 seconds faster traveling 1 km had it traveled at 75 km per hour. Since 2 sec = 1/1800 hour, we can create the following equation:

1/75 + 1/1800 = 1/r

Multiplying the entire equation by 1800r, we have:

24r + r = 1800

25r = 1800

r = 72

Answer: B

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by Brent@GMATPrepNow » Wed Dec 13, 2017 7:23 am
beater 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
Let's start with a word equation.

GIVEN: 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.
Let x = the car's speed

We can write: (travel time at x km per hour) = (travel time at 75 km per hour) + 2 seconds
Since the speeds are in km per HOUR, let's rewrite 2 seconds as 2/3600 hours (since there are 3600 second in one hour)

We get: (travel time at x km per hour) = (travel time at 75 km per hour) + 2/3600 hours
Or: (travel time at x km per hour) = (travel time at 75 km per hour) + 1/1800 hours

Travel time = distance/spee

We get: 1/x = 1/75 + 1/1800 hours
The least common multiple of the denominators is 1800x, so....
Multiply both sides by 1800x to get: 1800 = 24x + x
Simplify: 1800 = 25x
Solve: x = 1800/25 = 72

Answer: B

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