The radius of X 32  nuclide is measured to be twice the radius of B 49e. The number of neutrons in X is

radius of the nucleus is directly proportional  A13 where A is the mass no of the element.r1r2=A1A213    then r1r23=A1A2as r1=2r2       8=A1A2=A19      A1=72       72= np+nn=30+nn   no of neuron nn is 42