Suppose that N molecules are released at the origin (in 3D) at the time t = 0. If you are observing at some position r = (x, 0, 0), initially there will be no molecules there, but after some time the concentration will increase due to diffusion, reach a maximum, and then decrease again. 

a. Find the time when the concentration at r reaches its maximum. 

b. Calculate this maximum concentration value.