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A driver completed the first 20 miles of a 40-mile trip at

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Joined: 02 May 2005
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A driver completed the first 20 miles of a 40-mile trip at [#permalink]

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New post 01 Dec 2005, 13:31
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A driver completed the first 20 miles of a 40-mile trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40-mile trip? (Assume that the driver did not make any stops during the 40-mile trip.)

A. 65 mph
B. 68 mph
C. 70 mph
D. 75 mph
E. 80 mph

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New post 01 Dec 2005, 14:16
D.

if distance of two parts in a journey is same:
- first part abg speed s1
- second part ang speed s2

whole journey(two parts) avg speed = 2*s1*s2/(s1+s2)

put values to get:
60 = 2*50*x/(50+x)
solve for x
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New post 01 Dec 2005, 17:28
D
I am not sure if I did this the easiest way, but here's how I went about it.

40 miles/60 mph = average trip time
20 miles/50 mph = time it took to complete first 20 miles
20 miles/X mph = time it took to complete second 20 miles
therefore
40/60 hours = 20miles/50mph + 20miles/X mph.
Solved for X and you get 75

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New post 01 Dec 2005, 17:53
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time for 1st 20 miles = 20/50 = 2/5 hr
time for 2nd 20 miles = x

60 = 40/(x+2/5)
60x + 24 = 40
x = 40-24/60
x = 4/15hr

Average speed = 20/(4/15) = 75mph

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Re: A driver completed the first 20 miles of a 40-mile trip at [#permalink]

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New post 07 Oct 2016, 12:41
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Re: A driver completed the first 20 miles of a 40-mile trip at [#permalink]

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New post 16 Nov 2017, 00:43
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: A driver completed the first 20 miles of a 40-mile trip at   [#permalink] 16 Nov 2017, 00:43
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