Answer:
Wb = Vb ρb weight of block = volume block * density block
Ww = Vs * ρw weight of water displaced = volume block submerged * density of water
Vb ρb = Vs * ρw Archimedes Principle
Vs / Vb = ρb / ρw = .8
.8 of block = Vs / Vb .8 of block or 16 cm is submerged