<PRE><B>Question 177594</B>
Junior drove his rig on the Interstate 10 from San Antonio to El Paso. At the
 halfway point he noticed that he had been averaging 80 mph, while his company
 requires his average speed to be 60 mph. What must be his speed for the last
 half of the trip so that he will average 60mph for the entire trip.
;
let s = speed required on the last half of the trip
let d = half the distance of the trip
:
Write a time equation: Time = {{{dist/speed}}}
:
{{{d/80}}} + {{{d/s}}} = {{{(2d)/60}}}
:
{{{(sd+80d)/(80s)}}} = {{{(2d)/60}}}
cross multiply
60(sd + 80d) = 80s(2d)
:
60sd + 4800d = 160sd
:
divide eq by d
60s + 4800 = 160s
:
4800 = 160s - 60s
:
4800 = 100s
s = {{{4800/100}}}
s = 48 mph to average out to 60 mph
:
;
check solution using 600 mi from SA to EP (2d)
{{{300/80}}} + {{{300/48}}} = {{{(600)/60}}}
 3.75 + 6.25 = 10
</PRE>