Work,power,energy-mechanics maths

A motor car of mass 500 kg is moving along a straight horizontal road. The power of the engine is 40 kW & there is a constant resistance of magnitude 1000 N. At time t s the speed, in m/s, of the car is v. Show that dv/dt = (2(40-v))/v. Find, to the nearest tenth of a second, the time taken for the speed to increase from 10 m/s to 20 m/s.