Verified answer

That depends on the temperature!

Oh well. Density of air at 20C and 1 atm is

1.21 kg/m^3, or 0.0340 kg/ft^3,

so the total mass would be

13.5 x 19.5 x 8.0 x 0.0340 kg,

use a calculator,

might be around 60 kg.

If you INSIST on using non SI units it is YOUR job to be able to convert to and from those units.

You would know that the definition of the inch is EXACTLY 2.54 cm

So that a foot is 2.54 * 12 cm

You should know that a litre is a cubic decimetre. ie a cube 10 cm * 10 cm * 10 cm

so find the size of the room in dm

13.5 * 2.54*12/10

19.5 * 2.54 * 12/10

8 * 2.54 * 12/10

Multiply these to get the volume of the room in dm^3 ( Litres)

then multiply by the mass of each litre ( 1.19 g/L or 0.0019 kg/litre )

13.5 * 2.54*12/10 * 19.5 * 2.54 * 12/10 * 8 * 2.54 * 12/10 * 0.0019 ~= 113 kg