SOLUTION: Say I ride my bike 30 miles at an average speed of 17 mph. If I ride at an average speed of 14 mph for the first 15 miles how can I calculate the average speed for the final 15 mil

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Question 875111: Say I ride my bike 30 miles at an average speed of 17 mph. If I ride at an average speed of 14 mph for the first 15 miles how can I calculate the average speed for the final 15 miles?
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
Say I ride my bike 30 miles at an average speed of 17 mph.
If I ride at an average speed of 14 mph for the first 15 miles,
how can I calculate the average speed for the final 15 miles?
:
Write a time equation, Let s = speed required for the final 15 mi
Time = dist/speed
15%2F14 + 15%2Fs = 30%2F17
multiply equation by 14*17*s = 238s
238s*15%2F14 + 238s15%2Fs = 238s30%2F17
cancel the denominators
17s(15) + 238(15) = 14s(30)
255s + 3570 = 420s
3570 = 420s - 255s
3570 = 165s
s = 3570/165
s = 21.6 mph hr required on the last 15 mi
:
:
See if that checks out, find the times
15/14 + 15/21.6 = 30/17
1.07 + .69 = 1.76