A DC shunt motor has an armature of resistance 0.5 Ω and takes an armature current of 20 A when connected to a 220 V supply and running at 750 rev/min. What resistance should be connected in series...

A DC shunt motor has an armature of resistance 0.5 Ω and takes an armature current of 20 A when connected to a 220 V supply and running at 750 rev/min. What resistance should be connected in series with the armature to reduce the speed to 500 rev/min?