Question 791282: A car travels along a certain road at an average speed of 40 km/h, and returns along the same road at an average speed of 60 km/h. Calculate the average speed for the round trip. (Don't say 50 km/h!)
Found 3 solutions by Alan3354, solver91311, jim_thompson5910:
Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website! A car travels along a certain road at an average speed of 40 km/h, and returns along the same road at an average speed of 60 km/h. Calculate the average speed for the round trip. (Don't say 50 km/h!)
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Avg speed for a round trip = 2*r1*r2/(r1 + r2)
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2*40*60/(40+60)
48 mi/hr
This is true regardless of the distance.
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Answer by solver91311(24713)  (Show Source):
Answer by jim_thompson5910(35256)  (Show Source):
You can put this solution on YOUR website! when the speed is 40 km/hr, we know this much
d = rt
d = 40t
t = d/40
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when the speed is 60 km/hr, we know this much
d = rt
d = 60t
t = d/60
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So it takes d/40 hours when the speed is 40 km/hr and it takes d/60 hours when the speed is 60 km/hr
Add up the two times
d/40 + d/60 = 3d/120 + 2d/120
d/40 + d/60 = 5d/120
d/40 + d/60 = d/24
So overall, it will take d/24 hours (for the entire round trip)
For the sake of simplicity, let's say d = 24 (so that d/24 turns into 24/24 = 1)
Because going one way will have you going 24 km, this means that completing a full round trip will have you traveling 2*24 = 48 km
So because you have traveled 48 km in one hour (this is average over the entire trip), the average speed is 48 km/hr
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