The power dissipated in the circuit

$P = {{{V^2}} \over {{R_{eq}}}}$    ...(i)

v = 10 volt

${1 \over {{R_{eq}}}} = {1 \over R} + {1 \over 5} = {{5 + R} \over {5R}}$

${R_{eq}} = \left( {{{5R} \over {5 + R}}} \right)$

P = 30 W
Substituting the values in equation (i)

$30 = {{{{\left( {10} \right)}^2}} \over {\left( {{{5R} \over {5 + R}}} \right)}}$

${{15R} \over {5 + R}} = 10$

15R = 50 + 10R

5R = 50

R = 10 $\Omega$