I'm not quite sure how to approach this question, could someone help me out here? I'm sure I'll have further questions after this, so check back in this thread for updates. Anyways, the question asks;

A certaing brand of medicine tablet is in the shape of a sphere with diameter 5mm. Ther rate at which the pill dissolves is dr/dt = -k, where r is the radius of the sphere at time t hours, and k is a positive constant.

a) Show that r = 5/2 - kt

b) If the pill dissolves completely in 12 hours, find k.