Question 1164958
qmy drove to a friends house at a rate of 45mph. She returned by the same route at a rate of 50mph. The driving time for the round trip was 4hrs. what is the total distance amy travelled.
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Avg speed of the round trip = 2*45*50/(45+50) = 900/19 mi/hr
RT distance = r*t = (900/19)*4 = 3600/19 miles
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It's not possible to remember a formula for everything, but for a round trip:
Avg speed = {{{2*r[1]*r[2]/(r[1]*r[2])}}}
is similar to parallel work, parallel resistors, and easy to remember (IMO).