I am having trouble figuring out the stratagy required to figure out the following two related questions:

A pill is designed to be put under the tongue to be absorbed directly into the surrounding tissue. The pill can be considered a cylinder 5mm in diameter and 2mm thick. Initially the drug concentration is 3.2 mg/cm^-3.

A) if it takes 20 minutes for the concentration of the drug at the centre of the pill to reach 20% of its initial value, how long will it take for it to reach 10% of the initial value? It is to be assumed that the concentration of the drug in saliva at the surface of the pill is zero.

B) when the pill is inserted, what is the INITIAL mass transfer rate? The diffusivity of the drug through the 0.1mm boundary layer of saliva is 3*10^-6 m^2s^-1. Assume the concentration of the drug in saliva is the same as the pill at the interface.

From the context of the question, I believe it also is assuming the size of the pill remains the same and just the concentration of the drug is decreasing in the pill, or is that wrong?

Thanks for any help on how to tackle this problem.