Let $F_V$ be the viscous force and $F_B$ be the buoyant force acting on the ball.

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Then, when body moves with constant velocity,

$F_V+F_B=Mg$   

Or, $F_V=Mg-F_B$

$F_V={\rho }_{\mathrm{B}}\mathrm{Vg}-{\rho }_{\mathrm{L}}\mathrm{Vg}$ $=\left({\rho }_B-\right)\left({\rho }_L\right)Vg$, where, $V$ is the volume of small ball.

$=\left(8-1.3\right)\times {10}^3\times \frac{0.3\times {10}^{-3}}{8\times {10}^3}\times 10$

$=\frac{6.7\times 0.3}{8}\times {10}^{-2}$

$=\frac{67\times 3}{8}\times {10}^{-4}=25.125\times {10}^{-4}=25\times {10}^{-4} \mathrm{N}$

Thus, the value of $x=25$.