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

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Intern
Joined: 02 May 2005
Posts: 12
A driver completed the first 20 miles of a 40-mile trip at  [#permalink]

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01 Dec 2005, 14:31
1
00:00

Difficulty:

(N/A)

Question Stats:

100% (01:46) correct 0% (00:00) wrong based on 73 sessions

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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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VP
Joined: 22 Aug 2005
Posts: 1082
Location: CA

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01 Dec 2005, 15:16
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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Manager
Joined: 20 Nov 2005
Posts: 52
Location: Indianapolis, IN

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01 Dec 2005, 18:28
1
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
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 4869
Location: Singapore

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01 Dec 2005, 18:53
7
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
Non-Human User
Joined: 09 Sep 2013
Posts: 8532
Re: A driver completed the first 20 miles of a 40-mile trip at  [#permalink]

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16 Nov 2017, 01:43
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Re: A driver completed the first 20 miles of a 40-mile trip at &nbs [#permalink] 16 Nov 2017, 01:43
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