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A and S drive on the highway, going the same direction,

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bmwhype2
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A and S drive on the highway, going the same direction, [#permalink] New post   07 Nov 2007, 15:12
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.



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Re: A and S drive on the highway, going the same direction, [#permalink] New post   07 Nov 2007, 15:34
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


S is traveling @80 he is 4Km behind
A is traveling @72
To catch A in 20 mins
S needs to travel same distance as A + 4km

Distcnce = speed*time

Assume S accelarates by x

S's new spped = 80+x A's speed 72
Total time to catch 20 mins = 20/60 hrs=1/3hr

Distcnce traveled by S in 20 mins (80+x)*(1/3)
Distcnce traveled by A in 20 mins (72)*(1/3)

Difference of thier distcne should be 4

(80+x)*(1/3) - (72)*(1/3)=4
8+x=4*3
x=4 Answer D
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   07 Nov 2007, 17:08
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


A: can run 24 km in 20 minuets. so the distance for S is 24+4 = 28 km.

S: speed = 28/20 x 60 = 84.

so it is 4km/h
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   18 Dec 2007, 13:59
Damager wrote:
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


S is traveling @80 he is 4Km behind
A is traveling @72
To catch A in 20 mins
S needs to travel same distance as A + 4km

Distcnce = speed*time

Assume S accelarates by x

S's new spped = 80+x A's speed 72
Total time to catch 20 mins = 20/60 hrs=1/3hr

Distcnce traveled by S in 20 mins (80+x)*(1/3)
Distcnce traveled by A in 20 mins (72)*(1/3)

Difference of thier distcne should be 4

(80+x)*(1/3) - (72)*(1/3)=4
8+x=4*3
x=4 Answer D


thanks. very helpful!

OA is 4
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   18 Dec 2007, 14:09
Speed to cover 4 Km in 20 minutes = 4*60/20 = 12 Km/Hr
Difference between speed of S and A = 80 -72 = 8 Km /Hr

So actual increase in speed required = 12 - 8 = 4 Km/Hr.
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   18 Dec 2007, 18:38
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


Speed covered by A in those 20 mins = 1/3 * 72= 24 kms

B needs to cover 24 + 4 = 28 kms in 20 mins.

if a car travels 28 kms in 20 mins then in 60 mins it covers = 84 kms.

so S needs to travel at 84 kms..

so 4!!!
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   18 Dec 2007, 19:42
Answer is 4 km/h:

A in 30 min will travel: 72 * 1/3 = 24 km
S should travel 24 + 4 = 28 km in 20 min --> S's speed should be 28 / 1/3 = 84, so he should increase his speed by 4 km/h
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   18 Dec 2007, 20:11
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


My way . Let speed be increased by x
New 72 + x
Relative Speed = 80 - (72+x )
80 - ( 72 + x ) * 20 /60 = 4
x = 4 kmh
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   05 Feb 2008, 09:15
dkverma wrote:
Speed to cover 4 Km in 20 minutes = 4*60/20 = 12 Km/Hr
Difference between speed of S and A = 80 -72 = 8 Km /Hr

So actual increase in speed required = 12 - 8 = 4 Km/Hr.

thanks. this is much more intuitive and faster :-D
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   05 Feb 2008, 09:41
bmwhype2 wrote:
A and S drive on the highway, going the same direction, respective rates of 72 and 80 kmh. If S is 4km behind Andrew, by how much does he have to increase his speed to catch up with A in 20 minutes?

1 kmh
2 kmh
3 kmh
4 kmh
5 kmh

Can someone explain this using WORDs and then NUMBERS? i have never come across required acceleration in a RT=D question.


R 72+x 80
T 1/3 1/3
D d+4 d

24+x/3=80/3+4 ---> x/3= 80/3-72/3 +4 --> 8/3+12/3 --> 20/3=x/3 --> x=20 ????


O wait oops wrong person =)

R 72 80+x
T 1/3 1/3
D d d+4

80/3+x/3=24+4 ---> 80/3+x/3=28 --> 80+x=84 --> x=4
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Re: A and S drive on the highway, going the same direction, [#permalink] New post   06 Feb 2008, 18:04
In 20 minutes, A travels 72*1/3 = 24 kms

Therfore gap that needs to be made up is 24+4=28 kms, and it has to be done in 20 minutes. This correlates to a speed of 28/20=1.4 km/min, and in 60 mins that is 1.4*60=84 km/h

so speed has to be increased by 84-80=4 km/h



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
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Re: A and S drive on the highway, going the same direction, [#permalink]
06 Feb 2008, 18:04
Moderator:
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Senior Moderator - Masters Forum
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