Question 1104927
Avg speed of a round trip is similar to parallel resistors, parallel flows, parallel work, etc.
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Avg = 2*r1*r2/(r1+r2)
= 2*60*40/100
= 48 mi/hr
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It's not 50 because more time is spent at 40 mi/hr than at 60 mi/hr.
I'm not in favor of memorizing a lot of formulas, but this is an easy one.