In the given circuit the internal resistance of the 18 V cell is negligible. If R_1=400 Ω, R_3=100 Ω and R_4=500 Ω and the reading of an ideal…

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