<PRE><B>Question 1190897</B>
Sam jogged to Larry&#39;s house at 8 miles per hour and borrowed Larry&#39;s scooter, riding back at 20 miles per hour. If the round trip took 7 hours, how far is it to Larry&#39;s house?  Write a distance equation 
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The average speed of a round trip is {{{2*r[1]*r[2]/(r[1]+r[2])}}} where r1 and r2 are the 2 speeds.
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= 2*8*20/(8+20) = 320/28 = 80/7 mi/hr
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d = r*t = (80/7)*7 = 80 miles RT, 40 miles each way
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The formula is similar to parallel work, parallel resistance, etc. and is easy to remember.

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