solution:
The general form of the equation would be A = P x (1/2) ^ (t/h) where A = final amount, P = start amount, t = time (here in days) and h = half-life (has to be same units as t)
a) A = 192 x (1/2) ^ (15/5) = 192 x (1/2) ^3 = 192 x 1/8 = 24 mg
