<PRE><B>Question 890036</B>
If Charlene biked up a hill at a speed of 12 miles per hour and came down the hill using the same route and averaged of 15 miles per hour for her round trip, then what was her rate (in miles per hour) coming down the hill? 
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Avg speed of a round trip:
Avg = 2*r1*r2/(r1 + r2)
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15 = 2*12*r2/(12 + r2)
180 + 15r2 = 24r2
180 = 9r2
r2 = 20 mi/hr
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