[UPSC CSE 1996] Thermodynamics (Q172, 20M)

To what extent the pressure in a cathode ray tube must be reduced so that 90\% of the electrons emitted by the cathode reach the anode which is at a distance of 20\text{ cm} from the cathode? The mean molecular radius of main constituent gases of atmosphere is 2\text{\AA} and the number of molecules at standard pressure (760\text{ mm of Hg}) is 3 \times 10^{25} per \text{m}^3.