<PRE><B>Question 532865</B>
Rodney Racer averages 90mph in the first lap of a race.
 How fast must he go in the second half in order to average 120mph for the whole race?
:
Let s = speed required to average 120 mph
d = one lap distance
:
Write a time equation; time = dist/speed
:
{{{d/90}}} + {{{d/s}}} = {{{(2d)/120}}}
reduce the third fraction
{{{d/90}}} + {{{d/s}}} = {{{d/60}}}
Multiply by 180s to get rid of the denominators, results
2ds + 180d = 3ds
simplify, divide by d
2s + 180 = 3s
180 = 3s - 2s
180 = s
He has to run 180 mph to average 120 for the two laps</PRE>