SOLUTION: 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. (

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): You can put this solution on YOUR website!

The time it takes for the first leg of the trip is , where time is in hours and distance is in km. The time for the return trip is then

The total time for the trip is the sum of the two quantities above:

Once you have a simplified expression for , you can use that to divide into the total distance, , to find the average rate for the entire trip.

John

Egw to Beta kai to Sigma
My calculator said it, I believe it, that settles it

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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