SOLUTION: A man goes at the speed of 5 km/hour from P to Q and returns at the speed of 3 km/hour from Q to P. The average speed of whole journey is

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Question 830714: A man goes at the speed of 5 km/hour from P to Q and returns at the speed of 3 km/hour from Q to P. The average speed of whole journey is
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
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A man goes at the speed of 5 km/hour from P to Q and returns at the speed of 3 km/hour from Q to P.
The average speed of whole journey is:
:
Let a = average speed for the whole journey
Let d = the distance one way
:
Write a time equation, time = dist/speed
d%2F5 + d%2F3 = %282d%29%2Fa
multiply equation by 15a, cancel the denominators and you have:
3ad + 5ad = 15(2d)
8ad = 30d
divide both sides by d
8a = 30
a = 30/8
a = 3.75 mph