<PRE><B>Question 875111</B>
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
{{{15/14}}} + {{{15/s}}} = {{{30/17}}}
multiply equation by 14*17*s = 238s
238s*{{{15/14}}} + 238s{{{15/s}}} = 238s{{{30/17}}}
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 
</PRE>