Number of gas molecules in a room?

OK say you have this question in an interview.

I would make an estimate of the room, say for an example 6x6x2.5 = 90m^3

Then rearragning the ideal gas eq:

n = pV/RT

Pressure = 1atm (estimation)
Temperature = 298K (estimation)
R = 8.31 (forgot the units)
Volume = 90m^3

Therefore:

n = (1 x 90)/(8.31 x 298)
n = 0.04

Then multiply by avagadros constant:

No of Molecules = 0.04 x 6x10^23 = 2.4x10^22

OK so that is all fine, but if I try to work out the number of moles by a different method I get a completely different answer. Using the assumption that 1mole of gas occupies 24dm^3. If we convert the 90m^3 volume of the room into dm^3 we get 90000dm^3 and therefore 90000/24 = 3750mol, which cannot be right?

What am I doing wrong, this is bugging me a lot. :woo: