A gas effuses into a vacuum through a small hole of area A. The particles are then collimated by passing through a very small circular hole of radius a, in a screen a distance d from the first hole. Show that the rate at which particles emerge from the second hole is 41nA⟨v⟩(a2/d2), where n is the particle density and ⟨v⟩ is the average speed. (Assume that no collisions take place after the gas effuses through the second hole, and that d≫a.)