A cyclist bikes x distance at 10 mph and returns over the same path at 8 mph. What is...

  • Thread starter Thread starter Missy
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Missy

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...the cyclists average? OKAY my test says the answer is 8.9.

Wouldn't it be 9 because 10 + 8 = 18 ... divide it by 2 and you get 9... right???? what am i missing here????
 
x/t1 =10
x/t2 = 8
t1 =x/10
t2 = x/8
t1+t2 = (x/10) +(x/8) = 9x/40
average = 2x/(t1+t2)
2x / (9x/40)
80x /9x
80/9
more or less 8.9
 
Choose a distance, say 10 miles. Going out takes 60 min. Coming back takes 10/8 hours, or 75min. So the total time is 135 min for 20 miles, which equates to 8.9 mph.
 
speed = distance / time
10 = x / t(go)
t(go) = x / 10

8 = x / t(return)
t(return) = x / 8

t(total) = (x / 10) + (x / 8) = 18x / 80 = 9x / 40
distance(total) = 2x
speed(average) = distance(total) / time(total)
average = 2x / (9x / 40) = 80x / 9x = 80 / 9 = 8.888888...
 
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