<PRE><B>Question 712312</B>
If you must average 60 km/h for a 90-km trip, and you only average 40 km/h for the first 40 km, what velocity must you average the rest of the way?
:
Find the time required for the trip:
{{{90/60}}} = 1.5 hrs
:
Let s = velocity required for the last 50 km
:
40 km time + 50 km time = 1.5 hrs
{{{40/40}}} + {{{50/s}}} = 1.5
1 + {{{50/s}}} = 1.5 
subtract 1 from both sides
{{{50/s}}} = .5
.5s = 50
s = 100 km/hr required for the last 50 km </PRE>