Recall the proper relation between power and focal length, P=f100⇒f=P100=10100=10cm, now the relation between focal length and radius of curvature and refractive index, of the spherical surface lens,
f=2(μ−1)R (for equiconvex lens)
⇒10=2(μ−1)10⇒(μ−1)=21⇒μ=21+1=23.