DC Machines Problem

A 2kW series DC motor has an armature and field resistance of 0.16 W. If the power output from the motor is 1600W at 1200 rpm, with an armature current of 25 A and a supply voltage of 72 V, calculate:
(1). The torque and emf constants for this motor
(2). If the supply has a maximum power rating of 2000W, calculate the maximum torque available from the motor when taking the maximum power available from the supply.
(3). Calculate the efficiency of the motor at this maximum value of torque
(4). Calculate the new speed of this motor if the motor torque is reduced by 80%.