Current Electricity: JEE Main 2022 Physics question

A potentiometer wire of length $300 \mathrm{~cm}$ is connected in series with a resistance 780 $\Omega$ and a standard cell of emf $4 \mathrm{V}$. A constant current flows through potentiometer wire. The length of the null point for cell of emf $20\, \mathrm{mV}$ is found to be $60 \mathrm{~cm}$. The resistance of the potentiometer wire is ____________ $\Omega$.