4.8 (modified to match the material actually taught): It was written in one of the parts of the book

4.8 (modified to match the material actually taught): It was written in one of the parts of the book which we skipped that in 1D diffusion, if a large number of molecules are released at origin, then an observer sitting at a fixed point xA????10 sees a transient pulse of concentration pass by. Recall the solution of the diffusion equation for point release in 3D and assume there are million particles. Then show that the concentration measured by an observer at distance r from the initial release point will peak at certain time. Find this time in terms of r and D. Show that the concentration measured at that time is a constant times r -3 and find that constant.