DC MACHINE, SPEED CONTROL CALCULATION

Hi guys please help.

I am busy with speed control calculation on the DC(direct current) Machine.

QUESTION

A SERIES MOTOR IS FED FROM A 500V SUPPLY AND TURNS AT 800r/min at full load, while the current taken is 120A. THE FIELD AND THE ARMATURE RESISTANCES ARE 0,04 OHM AND 0.11 OHM RESPECTIVELY. IF IT IS ASSUMED THAT THE FIELD IS UNSATURATED, CALCULATE: THE SPEED WHEN THE TORQUE HAS 50% OF ITS FULL-LOAD VALUE, WHILE THE APPLIED VOLTAGE IS 500V AND A RESISTANCE OF 0.08 OHM IS CONNECTED IN PARRALLEL WITH THE FIELD. (ANSWER 1397 r/min)