During a trip, F traveled x% of the total distance

Brent@GMATPrepNow wrote:

vishal.pathak wrote:During a trip, F traveled x% of the total distance at an avg speed of 40 km/hr and the rest of the distance at an avg of 60 km/hr. In terms of x, what was F’s avg speed for the trip
a. (180-x)/2
b. (x+60)/4
c. (300-x)/5
d. 600/(115-x)
e. 12000/(x+200)
OA E
I tried to solve it using assumption. Please point out my mistake
Let the distance be 480 and let 25% (This is x) of 480 be traveled at 40km/hr and the rest at 60 km/hr
So dist traveled at 40 = 120. Therefore time = 3hrs
Dist traveled at 60 = 360. Therefore time = 6 hrs
Avg speed = 480/9 = 160/3
Then I put x = 25 in all options
My answer didn't match any of the choices.
[spoiler]E= 12000/(25+200) = 12000/225 = 800/9
[/spoiler]
Please help

Regards,
Vishal

Almost there, Vishal!
You had the correct answer right up until the end (see above, behind the spoiler).
To simplify 12000/225, we can first divide top and bottom by 25 to get: 480/9, which equals 160/3

Cheers,
Brent