Question 1185644
Becky walks to the stadium which is 580m way from her house for 20min . She walks back to her house using the same route at a speed of 20m per minute. 
What is her average speed. 
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Avg speed going is 580/20 = 29 m/min
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Avg speed for round trip = 2*29*20/(29+20) =~ 23.67 m/minute
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I don't recommend memorizing formulas generally, but the avg speed for a round trip is 2*v1*v2/(v1+v2).
This is the same as for parallel flows, parallel resistors, parallel work, etc. except for the factor of 2.