A small spherical bead is used as a controlled drug release capsule in the gastrointestinal system (i.e., your stomach). In this particular case, a 0.10-cm diameter bead has a uniform initial concentration of 0.20 mmole/L of the drug griseofulvin (species A). The diffusion coefficient of griseofulvin within the bead material is 1.5 × 10^-7 cm²/s. Upon release from the bead, the drug is immediately consumed so that the surface concentration is essentially zero. 

a. Using the concentration-time charts, determine the time it will take for concentration of the griseofulvin in the center of the bead to reach 10% of its initial value. 

b. Using the infinite series analytical solution for c_A(r,t), determine the time it will take for concentration of the griseofulvin in the center of the bead to reach 10% of its initial value, and then plot out the concentration profile from r = 0 (center) to r = R (surface). Carry out your calculations on a computer spreadsheet.