A 250. V, D.C shunt motor has an armature current of 20 A when running at 1000 r.p.m against some load torque. The armature resistance is 0.5 Ω and brush contact is 1 volt per brush. By how much must the flux be reduced (in %) to raise the speed by 50%. If the developed torque is constant? Ignore effects of armature reaction and magnetic saturation.