The radius of the nucleus of an atom whose mass number is 125 is

Radius of a nucleus of atomic number of A will be given by $r_A=A^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}{r}_H$

For A=125, $r_{125}={125}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}\times \left(1.2\times {10}^{-15}\right)=6\times {10}^{-15} m$