A 0.82 kg piece of wood floats in water but is found to sink in alcohol (sg=0.79), in which it has an apparent mass of 0.059 kg.

Upthrust = loss in weight = 0.82 - 0.059 = 0.761 Kg

upthrust = weight of the displaced fluid = 0.79*Vg where Vg is the volume of the piece of wood

Therefore:

0.761 = 0.79 Vg

Vg = 0.96329

Density of wood (sg of wood) = mass / volume

density of wood = 0.82 / 0.96329 = 0.85