\(R_{\mathrm{eq}}=\frac{5 \,R}{5+R}\)
Power dissipated in the given circuit is
\({P=\frac{V^{2}}{R_{\mathrm{eq}}} \text { or } 30=\frac{(10)^{2}}{\left(\frac{5 \,R}{5+R}\right)}} \)
\({150\,R=100(5+R)}\)
\({150\, R=500+100 \,R \quad \text { or } 50 \,R=500} \)
\({R=\frac{500}{50}=10 \,\Omega}\)