The velocity of a small ball of mass M and density ρ , when dropped...

When velocity becomes constant then,

${F}_{v} + {F}_{b} = Mg$

${F}_{v} = Mg - {F}_{b}$

$= V{ρ}_{b}g - V{ρ}_{g}g$

$= Vg\left[ρ - \dfrac{ρ}{2}\right]$

$= \dfrac{Vgρ}{2}$

$= \dfrac{( Vρ ) g}{2} = \dfrac{Mg}{2}$