So this guy will have to push its way to the other side. And by pushing, I mean colliding. So it's going to travel some distance A, then distance B, then CDEF, and finally G.
So each time it collides and it bounces off. Now, to find the mean free path or the average distance between a two collisions, I simply add up all the distances up and divide by the number of collisions. And that's my mean free path. Now, because of this, we can use Grams law to approximate diffusion and we'll see why in a second.