The focal length of the plano-convex lens is $1/f = (n-1)(1/R)$. When the plane side is silvered, it acts as a combination of a lens and a plane mirror. The effective focal length F of the combination is $1/F = 2/f + 1/f_m$. Since $f_m = \infty$, $F=f/2$. The equivalent mirror has a radius of curvature $R_{eq} = 2F = f = R/(n-1)$.