Before revision: So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules (without vapour molecules) in the volume must decrease by the same number for the pressure to remain constant without applying a change in temperature .
After revision: Therefore, when some number N of water molecules (vapor) is introduced into a volume of dry air, the number of air molecules in that volume must decrease by the same number for the pressure to remain constant without usingN a change in temperature .