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OG #12 PS #149 Help

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OG #12 PS #149 Help

by Niner710 » Wed Nov 25, 2009 7:53 pm
I have a question on PS #149 on OG12

During a trip, Francine traveled x percent of the total distance at an average speed of 40 mph and the rest of the distance at an average speed of 60 miles per hour. In terms of x, what was Francine's average speed for the entire trip?

Answer (E)
12,000/x +200

I don't understand the answer explanation. How can you represent the distance as x%? Can someone explain. Thanks.
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by aspirant1 » Wed Nov 25, 2009 9:26 pm
Lets assume the total distance = 1 mile
x% of total miles = x/100
Distance/ speed = time
time = x/4000

rest of distance
time = (100-x)/60*100

Total time = (x/4000)+(100-x)/6000 = (x+200)/12000

So, total average speed = 1/((x+200)/12000) = 12000/(x+200)
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