A girl rides a bicycle that is powered by an electric motor. A battery transfers energy to the electric motor. The emf of the battery is 16V and it can deliver a charge of 43kC when discharging completely from a full charge. The maximum completely from a full charge. 

The maximum speed of the girl on a horizontal road is 7.0m/s with energy from the battery alone. The maximum distance that the girl can travel under these conditions is 20km. 

Show that the time taken for the battery to discharge is about 3 x 10^3 s.