Average Speed from ManhattanGmat

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Average Speed from ManhattanGmat

by jcmsolis » Mon Dec 10, 2007 12:23 pm
Average speed

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Martha takes a road trip from point A to point B. She drives x percent of the distance at 60 miles per hour and the remainder at 50 miles per hour. If Martha's average speed for the entire trip is represented as a fraction in its reduced form, in terms of x, which of the following is the numerator?

(A) 110
(B) 300
(C) 1,100
(D) 3,000
(E) 30,000

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by Argen » Mon Dec 10, 2007 3:08 pm
The answer is D.

Let d be the total distance driven. So the total time is t = xd/60 + (1-x)d/50
And the average speed is d/t

This would simplify to 3000/(60-10x).

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by gmatguy16 » Mon Dec 10, 2007 3:37 pm
imo d,whats oa?

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by sirikesav » Tue Dec 11, 2007 1:54 am
I think answer is E. here is My explanantion.

Total distance = y

Total time = (XY/100)/60 + (100-x)y/50*100

Average Speed = Total distance /Total time

= 1/(x/6000 + 100-x/5000)

=30000/(5x + 600 - 6x)

= 30000/(600-x)

hence answer is E.Please Correct me if I am wrong

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by samirpandeyit62 » Tue Dec 11, 2007 2:56 am
I've not solved it though but I agree with sirikesav it should be E
Regards
Samir