Please help me with the sum.....

Dear student, Upthrust = ( V×ρw - m )×g ...............(1) where V is displaced volume of water, ρw is water density, m is mass of solid and g is acceleration due to gravity if we substitute vales for hollow ball-A, we get, (15×10-6 ×1000 - 12×10-3)×9.8 = 3×10-3 ×9.8 = 29.4×10-3 N volume of solid ball-B = 12×10-3 / 7800 = 1.54 × 10-6 m3 Hence upthrust for ball-B = (1.54×10-6× 1000 - 12×10-3 )×9.8 = -10.46×10-3 × 9.8 = -103 ×10-3 N The solid iron ball B will sink in water .As the upthrust is very lower on the solid ball than the upthrust on the hollow ball . Regards