Answer
$\varepsilon =33.33V$
positive terminal at the top
Work Step by Step
Since $300\Omega $ dissipates no power it means potential drop across $300\Omega$ is zero which means current through $300\Omega $ is zero. Also potential drop across $200\Omega $ will be equal to $\varepsilon$.
applying kirchofs law we have
$50V - 100I -200I =0$
$ I = \frac{1}{6}A$
now potential drop across $200\Omega $ is
$V = IR = \frac{1}{6}* 200 V= 33.33V$
hence value of $\varepsilon$ is
$\varepsilon =V =33.33V$
positive terminal at the top
