free path of CO under these new conditions is 52000 m ( = 5.2
x 104 m)
The mean free path of molecules in the gas phase is inversely proportional to the pressure , hence when the pressure of the gas is reduced by 5 decades from 10-4 Torr to 10-9 Torr, the mean free path will increase by a factor of 100000.
i.e. λ = 0.52 x 100000 = 5.2 x 104 m