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

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CEO
Joined: 29 Mar 2007
Posts: 2559
If the car had traveled 20kmph faster than it actually did, [#permalink]

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25 Oct 2007, 15:34
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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
Joined: 03 May 2007
Posts: 872
Schools: University of Chicago, Wharton School

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25 Oct 2007, 19:00
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
Joined: 29 Mar 2007
Posts: 2559

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25 Oct 2007, 21:08
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, 21:08
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