Question 1140236
You travel the first 48 miles of a journey at an average of 36 mph and the second 48 miles at an average of v mph. What must be v if you want to average 45 mph over the 96 mile journey?
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Avg speed of a round trip or of equal parts is:
Avg = 2*r1*r2/(r1 + r2)
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45 = 2*36*r2/(36+r2)
45*(36+r2) = 72r2
1620 + 45r2 = 72r2
27r2 = 1620
r2 = 60 mi/hr
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IMO, this is a formula you should remember.  It's similar to parallel work, parallel flow, parallel resistors, etc.  Only the factor of 2 is different.
You can remember it, or not.
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It's also the basis for the famous Michelson-Morley experiment that showed no effects of the "ether" and dispelled belief in the existence of the ether.  Ether is now stored in the same closet as phlogiston.