A potentiometer wire of length $10 \mathrm{cm}$ and resistance $32 \mathrm{\Omega }$ is connected in series with a battery of EMF $2.5 \mathrm{V}$, internal resistance $3 \mathrm{\Omega }$ and external resistance $R$. If a secondary cell of $0.4 \mathrm{V}$ is balanced at $8 \mathrm{cm}$ length. Then value of $R$ is