<PRE><B>Question 1147983</B>
24*40/(24+40) = 960/64 = 15 minutes
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Product over sum works for several types of problems.
Parallel flows
Parallel resistors
Parallel work
Maybe some others.
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Also, the average speed of a round-trip (or 2 legs of equal length) is similar.
If the speeds are R1 and R2, avg = 2*R1*R2/(R1 + R2)
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