(Solved): Consider two concentric spheres as shown in Figure 1. The sphere in the center is insulated by an en ...

Consider two concentric spheres as shown in Figure 1. The sphere in the center is insulated by an enveloping layer, 
component A. Eventually, the material envelope dissolves in a slow process of mass diffusion of the envelope materia with the medium, component B. At r=R1 there is a concentration of CA of the layerenvelope of CA=C1, and in the outer shell, that is, for r=R2, we have that CA=C2. 
Give a expression to determine the time in which the enveloping layer dissolves completely.