<PRE><B>Question 342957</B>
Plane flies roundtrip from Denver to San Francisco a total of 1600 miles. Trip time is 9 hours. Return trip from San Francisco, the plane averages 40 mph faster than the outbound flight. What is the average speed to San Fran and back?
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Is 9 hours the time for the round trip?  800 miles each way?  If so:
The average overall is 1600/9 =~ 177.8 mph
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If you want the avg speed for each half of the trip:
d = rt
t = d/r
9 = 800/r + 800/(r+40)
9*r*(r+40) = 800(r+40) + 800r
9r^2 + 360r = 800r + 3200 + 800r = 1600r + 3200
9r^2 - 1240r - 3200 = 0
*[invoke solve_quadratic_equation 9,-1240,-3200]
r = x (Ignore the negative solution)
r =~ 140 going
r + 40 =~ 180 returning
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