In the given circuit the internal resistance of the 18 V cells is negligible. If $\mathrm{R}_1=400 \Omega, \mathrm{R}_3=100 \Omega$ and $\mathrm{R}_4=500 \Omega$ and the reading of an ideal voltmeter across $\mathrm{R}_4$ is 5 V , then the value of $R_2$ will be:
