Question

what is the mass of air in the room with dimension 5 m by 4 m by 3 m,if the density of air is 1.29 kg/m

Answer 1

____________________________________________________________
density = mass / volume 

mass = 5 x 4 x 3  x 1. 29

 mass = 77.4 kg 
___________________________________________________________

Answer 2

Volume of the room is

V = 5×4×3 = 60 m^3

Now, Density = Mass/Volume

So Mass = Density×Volume
So Mass = 1.29 × 60
So, Mass = 77.4 kg

Thus, mass of air in the room is 77.4 kg