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How much time did it take a certain car to travel 400

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How much time did it take a certain car to travel 400 [#permalink] New post 25 Jan 2008, 05:56
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How much time did it take a certain car to travel 400 kilometers?

(1) The car traveled the first 200 kilometers in 2.5 hours.

(2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.

The OA is B. Can someone show how to solve this using the equation so that I can understand how option B is suff?
thanks
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Re: DS: Rate [#permalink] New post 25 Jan 2008, 06:16
Expert's post
(V+20)(t-1)=400

Vt=400

From second condition we can find t.
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Re: DS: Rate [#permalink] New post 25 Jan 2008, 06:19
Distance: 400
Speed: x
Time: Image

Image = Image + 1

Now solve:
Image

Image

Image

Image

so X is equal to 80 or -100. Obviously he can't be going -100km/h, so x = 80kmh.

Test it out:

Image

and

Image

Answer B

Hope that helps
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Re: DS: Rate [#permalink] New post 25 Jan 2008, 06:30
actually, where did you get this from:

400(x+20) = 400(x) + x(x+20)
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Re: DS: Rate [#permalink] New post 25 Jan 2008, 07:24
tarek99 wrote:
actually, where did you get this from:

400(x+20) = 400(x) + x(x+20)


to eliminate the denominators I multiplied each term by x(x+20). Some of the terms canceled out, such as 400/x * x(x+20) where the x cancels out and we're left with 400(x+20)
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Rates [#permalink] New post 06 May 2011, 09:30
How much time did it take a certain car to travel 400 kilometers?

1) The car traveled the first 200 kilometers in 2.5 hours.

2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.
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Re: Rates [#permalink] New post 06 May 2011, 10:16
1) Insufficient
We dont know the average speed. Hence the time, spent in first 200 km does not make any sense.

2) Sufficient
vt = 400 -------(1)

vt = 200 = (v+ 20) (t - 1)

vt = vt + 20t - v - 20
20t = v + 20

Multiply both sides by t
20t^2 = vt + 20t
20t^2 = 400 + 20t -----(we know vt = 400)
t^2 = 20 + t

hence t^2 - t - 20 = 0

This will gives us two roots one positive and one negative. Hence t has unique value. Sufficient

hence B
naveenhv wrote:
How much time did it take a certain car to travel 400 kilometers?

1) The car traveled the first 200 kilometers in 2.5 hours.

2) If the car's average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hour less time than it did.
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Re: Rates [#permalink] New post 06 May 2011, 10:22
let speed of the car = s
new speed = s+20

400/s - 400/(s+20) = 1
one known and one equation. It can be solved.

Hence B.

a. nothing provided for the rest half of the journey. not sufficient.
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Re: Rates [#permalink] New post 06 May 2011, 10:25
Please post it in data sufficiency section.this is problem solving thread.

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Re: Rates [#permalink] New post 06 May 2011, 10:29
v = 400/t

1. Not sufficient

As we don't know about next 200km

2. Sufficient

v+20 =400/(t-1)
=> 400/t + 20 = 400/(t-1)
=> sufficient to find t

Answer is B.

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Re: Rates   [#permalink] 06 May 2011, 10:29
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