For this case, we need to know the density of the air. Density is a physical property of a substance that represents the mass of that substance per unit volume. From the given values, it is calculated as follows:
Density = 13/10 = 1.3 kg/m^3
Mass of air in the room = 1.3 (4.2 x 3.5 x 2.6 ) = 49.69 kg