A $220$V DC series motor runs drawing a current of $30$ A from the supply. Armature and field circuit resistances are $0.4\Omega$ and $0.1\Omega$, respectively. The load torque varies as the square of the speed. The flux in the motor may be taken as being proportional to the armature current. To reduce the speed of the motor by $50$%, the resistance in ohms that should be added in series with the armature is ______. (Give the answer up to two decimal places.)