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How much time did it take a certain car to travel 400 [#permalink]

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25 Jan 2008, 05:56

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A

B

C

D

E

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(N/A)

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0% (00:00) correct
0% (00:00) wrong based on 3 sessions

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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

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)

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.

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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