ai) You cannot use P= F v as the velocity is not 30ms*-1 which is the maximum speed. The average speed should be 15ms*-1. So you would obtain 6s instead of 12 s.
Using power = energy divide by time taken will give the right answer which is 6s.
ii)reason 1: Some kinetic energy is loss in the form of heat in overcoming the frictional forces
reason 2: The car engine cannot achieve its maximum power at t=0s . Assume constant power is throughout the motion is not possible.
