calculate the density of the wood

Question

A bar of brass of mass 180g and density 8.5gm^-3 is attached to a piece of wood of mass 70g and both are totally immersed in water. the apparent mass is 120g. Calculate the density of the wood

Answer 1

First of all, it will be assumed that the density of the brass is 8.5 g/cm³ and not 8.5 g/m³. From the way the question is typed, one can't tell what you mean.  Having said that, the volume of water displaced by the brass alone would be 

180 g x 1 cm³/8.5 g = 21.18 cm³

If the apparent mass was determined by assuming a density of 1.0 g/cm³ for water, and 120 cm³ was displaced, then the volume displaced by the wood alone would be 120 cm³ - 21.18 cm³ = 98.82 cm³.  Since the mass of the wood is 70 g, the density of the wood is 70 g/98.82 cm³ = 0.708 g/cm³ = 0.71 g/cm³ (to 2 significant figures).