Let heavy nucleus breaks into two nuclei of mass m1 and m2 and move away with velocities V1 and V2 respectively. According to question, V2V1=278 m1V1=m2V2 (Law of momentum conservation) ⇒m2m1=V1V2=827ρ×34πR23ρ×34πR13(∵ density ρ= volume mass ) ⇒(R2R1)=(827)31=(23)3×31∴R2R1=23