Find the speed at which ammonia diffuses through air.?

Ammonia, which has a strong smell, is diffusing through air contained in a tube of length (L) and cross-sectional area of 5.70 x 10^-4 m^2. The diffusion constant for ammonia in air is 4.2 x 10 ^-5 m^2/s. In a certain time, 2.32 x 10^-8 kg of ammonia diffuses through the air when the difference in ammonia concentration between the ends of the tube is 4.53 x 10^-2 kg/m^3. Find the speed at which ammonia diffuses through air, that is, the length of the tube divided by the time to travel this length.