Multiply field strength (N/kg) by mass (kg) to get weight (N)
At the start, the car is carrying
4.7 kg * (9.8 N/kg) = 46.06 N
of fuel.
At the end, it is carrying
3.0 kg * (9.8 N/kg) = 29.4 N.
Assuming the car remains completely intact, its weight was reduced by
46.06 N - 29.4 N = 16.66 N.
