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