When boat is sunk into the liquid the net buoyancy on the boat is counterbalanced by weight of the boat
So here weight of the boat = Buoyancy force
let say boat is sunk by distance "h"
now we can say
[tex]F_b = \rho * V * g[/tex]
[tex]F_b = 1000*0.10 * 0.08 * h * 9.8[/tex]
now by above force balance equation we can write
[tex]m*g = F_b[/tex]
[tex]0.040 * 9.8 = 1000 * 0.10 * 0.08 * h * 9.8[/tex]
[tex]0.040 = 8h[/tex]
[tex]h = 5 * 10^{-3} m[/tex]
so boat will sunk by total 5 mm distance