confusion about the calculation of power

Question

A car of mass $1600$ kg is pulling a caravan of mass $800$ kg on a straight horizontal road. The car and the caravan, moving at a constant speed of $25$ m/s, are connected by a light rigid tow bar. The resistances to the motion of the car and caravan are $400$ N and $250$ N. To find the power of the car's engine, won't we only multiply $400$ with $25$? I don't understand why $250$ has to be added to $400$ since $250$ is not the driving force of the car. Please help.