Relative density is defined as:
dr = ds / dw
where:
dr: relative density
ds: density of the substance.
dw: density of water.
In this case we have the relative density of oak wood:
dr = 0.64.
We want to find the density of the substance: ds
Therefore we need to know the density of water in the cgs unit system:
dw = 1g / c ^ 3
Finally:
ds = dr * dw
ds = 0.64 * 1 g / cm ^ 3
ds = 0.64 g / c ^ 3
The density of the oak wood in cgs is 0.64 g / cm ^ 3