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.

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)   (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 hours cycling up the hill. She spent hours cycling down the hill. The full time is hours, in total. The dull distance is 2D. The average speed is . Cancel D in the numerator and in the denominator. You will get Average speed = = = = 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)   (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.
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Avg = 2*15*35/(15+35)
= 21 km/hr

Answer by stanbon(75887)   (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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