From P=Wt=Fst=Fv, traction:
f = Pv = 18× 103 w20m/s = 900n，
Because the car is driving at a constant speed, resistance and traction are a pair of balanced forces, and the two forces are equal in size.
So the resistance of the car f = f = 900 n;
(2) Work done by towing100km:
w = Fs = 900n× 100× 103m = 9× 107j，
Heat released by burning 7.5 liters of gasoline:
Q amplifier = VQ = 7.5l× 4×107j/l = 3×108j,
Efficiency of the engine:
η=WQ amplifier ×100% = 9×107j3x108jx100% = 30%.
Answer: (1) When the car is driving at a constant speed of 72km/h, the resistance of the car is 900 N;
(2) When the car is driving at a constant speed of 72km/h, the efficiency of the engine is 30%.