Question 885040: Find out the average speed of person who rides the first mile at the rate of 8 miles an hour, the next mile at the rate of 7.5 miles an hour and the third mile at the rate of 5.5 miles an hour.
Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website! Find out the average speed of person who rides the first mile at the rate of 8 miles an hour, the next mile at the rate of 7.5 miles an hour and the third mile at the rate of 5.5 miles an hour.
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total time = 1/8 + 1/7.5 + 1/5.5 hours
Total distance = 3
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General solution, using speeds of a, b & c:
time = 1/a + 1/b + 1/c = (ab + ac + bc)/abc hours
Distance = 3 miles
Avg speed = d/t = 3abc/(ab + ac + bc)
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Sub the numbers:
Avg = 3*8*7.5*5.5/(8*7.5 + 8*5.5 + 7.5*5.5)
= 990/(60 + 44 + 165/4) = 990/(581/4)
= 3960/581 mi/hr
=~ 6.8158 mi/hr
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Note: avg = 3abc/(ab + ac + bc) is the same as 3 resistors in parallel, times 3
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