SOLUTION: Please help me. Jenny is cycling up a hill at a constant speed of 15 km/h and returns down the hill at a constant speed of 35 km/h. Calculate the average speed of her whole trip.

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Question 1055299: Please help me.
Jenny is cycling up a hill at a constant speed of 15 km/h and returns down the hill at a constant speed of 35 km/h. Calculate the average speed of her whole trip.
Found 3 solutions by ikleyn, Alan3354, stanbon:
Answer by ikleyn(52756) About Me  (Show Source):
You can put this solution on YOUR website!
.
Please help me.
Jenny is cycling up a hill at a constant speed of 15 km/h and returns down the hill at a constant speed of 35 km/h.
Calculate the average speed of her whole trip.
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Let D be the distance in one direction.

Jenny spent D%2F15 hours cycling up the hill.

She spent D%2F35 hours cycling down the hill.

The full time is D%2F15+%2B+D%2F35 hours, in total.

The dull distance is 2D.

The average speed is %282D%29%2F%28D%2F15+%2B+D%2F35%29.

Cancel D in the numerator and in the denominator. You will get

Average speed = 2%2F%281%2F15+%2B+1%2F35%29 = 2%2F%28%2835%2B15%29%2F%2815%2A35%29%29 = 2%2A%2815%2A35%29%2F%2815%2B35%29 = 21 mph.

For more problems like this see the lessons
    - Calculating an average speed: a train going from A to B and back
    - One more problem on calculating an average speed
in this site.

Also, you have this free of charge online textbook in ALGEBRA-I in this site
    - ALGEBRA-I - YOUR ONLINE TEXTBOOK.

The referred lessons are the part of this textbook under the topic "Travel and distance".


Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
Jenny is cycling up a hill at a constant speed of 15 km/h and returns down the hill at a constant speed of 35 km/h. Calculate the average speed of her whole trip.
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Avg speed of a round trip is similar to parallel resistors and parallel flow or work.
---
Avg = 2*15*35/(15+35)
= 21 km/hr

Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
Jenny is cycling up a hill at a constant speed of 15 km/h and returns down the hill at a constant speed of 35 km/h. Calculate the average speed of her whole trip.
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Up the hill DATA:
rate = 15 km/h ; distance = x km ; time = x/15 hrs
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Down the hill DATA:
rate = 35 km/h ; distance = x km ; time = x/35 hrs
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Average speed = (total distance)/(total time) = (2x)/(x/15+x/35)
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= (2x)/[(35x+15x)/(15*35)]
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= (2x)/[(50x)/(525)]
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= (2x)
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= 25 km/hr
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Cheers,
Stan H.
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