As shown in the figure, a potentiometer wire of resistance $20 \Omega$ and length 300 cm is connected with resistance box (R.B.) and a standard cell of emf 4 V . For a resistance ' R ' of resistance box introduced into the circuit, the null point for a cell of 20 mV is found to be 60 cm . The value of ' R ' is $\_\_\_\_$ $\Omega$.