Velocity of the ball as it reaches water surface is 2ghNet upward force acting on the body F=ρVg−ρ′Vg⇒ma=ρVg−ρ′Vg⇒(ρ′V)a=Vg(ρ−ρ′)⇒a=g(ρ′ρ−ρ′) in upward directionlets say it sinks to the depth H, at this instant final velocity becomes v=0, so we have0=(2gh)2−2×g(ρ′ρ−ρ′)×H∵v2=u2+2 as⇒H=ρ−ρ′hρ′