For a given mass of water molecules, the density is less if volume is large. So the volume of the vapour is `1000//0.6=1//(6xx10^-4)` times larger. If densities of bulk water and water molecules are same, then the fraction of molecular volume to the total volume in liquid state is 1. As volume in vapour state has increased, the fractional volume is less by the same amount, i.e.` 6xx10^(-4)`.