An electric motor operates on 50V supply and carry currrent 12A. If the efficiency of the motor is 30% what is the resistance of the winding of the motor.

we know that the efficiency is given as

η  = output power /  input power =  P_out / P_in

here,

η = 0.3 [30%]

P_in = VI = 50 volts x 12 amps = 600 W

so,

P_out = 0.3 x 600 = 180 W

now,

power lost = dP = P_in - P_out =  I²R

so, winding resistance will be

R = (P_in - P_out) /  I²

or

R = (600 - 180) / 12²

thus, we get

R = 2.91 ohms