The speed of the car is only based on miles traveled and time to travel. 600
KM in 4.3 Hours is
600/4.3 = 139.5349 KM/Hr
Neither the weight of the load nor the amount of fuel used plays into speed,
not directly.
Quite often in 'word questions' information is given, such as the weight of
the passengers, that is NOT needed to solve the problem. This is included to
see if the student can pick out the pieces that ARE needed from among a lot
of information. So the fact that there was 60Kg of passengers might have
been mentioned just to see if you and he would ignore it or get confused by
it being there.
Also, be sure that it is 4.3 hours (4 hours 20 minutes) and not 4 hours and
30 minutes (4.5 hours). At 4.5 hours the speed would be
600/4.5 = 133.3333 KM/Hr