\(R \propto \sin (2 \theta)\)
\(\frac{ R _1}{ R _2}=\frac{\sin \left(2 \theta_1\right)}{\sin \left(2 \theta_2\right)}=\frac{\sin (2 \times 15)}{\sin (2 \times 45)}=\frac{\sin 30^{\circ}}{\sin 90^{\circ}}\)
\(\frac{50}{ R _2}=\frac{1}{2}\)
\(R _2=100\,m\)