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If the car had traveled 20kmph faster than it actually did,

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CEO
CEO
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Joined: 29 Mar 2007
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If the car had traveled 20kmph faster than it actually did, [#permalink] New post 25 Oct 2007, 14:34
If the car had traveled 20kmph faster than it actually did, the trip would have lasted 30minutes less. If the car went exactly 60km, what speed did it travel?



Here is my equation and I can't get to the right answer... Where am I making the mistake?

(R+20)(t-1/2)=60

Rt=60

Rt-1/2R+20t -10 =60 ---> Rt-1/2R+20t=70

60-1/2R+20t=70 ---> -1/2R+20t=10 (t=60/R)

-1/2R+20(60/R)=10 ---> 1200/R-R/2=10 --> (2400-R^2)/2R=10

2400-R^2=20R ---> R^2-20R-2400=0 (R-60)(R+40)=0

So R= 60... OA is 40.... what am I doing wrong?
Director
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Joined: 03 May 2007
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Schools: University of Chicago, Wharton School
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Re: Challenges Rate [#permalink] New post 25 Oct 2007, 18:00
GMATBLACKBELT wrote:
If the car had traveled 20kmph faster than it actually did, the trip would have lasted 30minutes less. If the car went exactly 60km, what speed did it travel?



Here is my equation and I can't get to the right answer... Where am I making the mistake?

(R+20)(t-1/2)=60

Rt=60

Rt-1/2R+20t -10 =60 ---> Rt-1/2R+20t=70

60-1/2R+20t=70 ---> -1/2R+20t=10 (t=60/R)

-1/2R+20(60/R)=10 ---> 1200/R-R/2=10 --> (2400-R^2)/2R=10

2400-R^2=20R ---> R^2 - 20R-2400=0
(R-60)(R+40)=0

So R= 60... OA is 40.... what am I doing wrong?


2400 - R^2 = 20R
R^2 + 20R - 2400 = 0
(R + 60)(R - 40)=0
R = 40
CEO
CEO
User avatar
Joined: 29 Mar 2007
Posts: 2585
Followers: 17

Kudos [?]: 242 [0], given: 0

Re: Challenges Rate [#permalink] New post 25 Oct 2007, 20:08
Fistail wrote:
GMATBLACKBELT wrote:
If the car had traveled 20kmph faster than it actually did, the trip would have lasted 30minutes less. If the car went exactly 60km, what speed did it travel?



Here is my equation and I can't get to the right answer... Where am I making the mistake?

(R+20)(t-1/2)=60

Rt=60

Rt-1/2R+20t -10 =60 ---> Rt-1/2R+20t=70

60-1/2R+20t=70 ---> -1/2R+20t=10 (t=60/R)

-1/2R+20(60/R)=10 ---> 1200/R-R/2=10 --> (2400-R^2)/2R=10

2400-R^2=20R ---> R^2 - 20R-2400=0
(R-60)(R+40)=0

So R= 60... OA is 40.... what am I doing wrong?


2400 - R^2 = 20R
R^2 + 20R - 2400 = 0
(R + 60)(R - 40)=0
R = 40



I knew it was something so simple, but I just couldn't snuff it out.

Thx
Re: Challenges Rate   [#permalink] 25 Oct 2007, 20:08
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If the car had traveled 20kmph faster than it actually did,

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