$R=\frac{U_z}{I_z}$

$P_z=U_z{I}_z|:U_z$

$I_z=\frac{P_z}{U_z}$

$R=\frac{U_z}{\frac{P_z}{U_z}}$

$R=\frac{U_z^2}{P_z}$

$R=\frac{{\left(230\mathrm{V}\right)}^2}{60\mathrm{W}}=\frac{52900{\mathrm{V}}^2}{60\mathrm{W}}\approx 882\frac{\frac{\cancel{\mathrm{W}}}{\mathrm{A}}\cdot \mathrm{V}}{\cancel{\mathrm{W}}}=882\Omega$

$\frac{R}{R_0}=\frac{882\cancel{\Omega }}{65\cancel{\Omega }}\approx 13,6$

$T\approx 2550\mathrm{K}$