What is the (approximate mass of air in a typical room with dimensions 5.4m×3.8m×2.5m?

The answer is 62.84 kg.

To calculate the mass of air (m) we need to use the volume (V) of the air (which is the same as the volume of the room) and the density (D) of the air.
D = m/V

The density of air is: D = 1.225 kg/m³
The volume of air is: V = 5.4m×3.8m×2.5m = 51.3 m³

D = m/V
m = D * V
m = 1.225 kg/m³ * 51.3 m³
m = 62.84 kg

Iam not sure , but probably 750 or 7.5