The problem is #12 on the attached document.
Suppose that the half life is t_half. The number of atoms in the sample as a function of time is then given by:
N(t) = N(0) 2^(-t/t_half)
The rate at which atoms are decaying is thus given by minus the derivative (decay rate equals the rate at which N is decreasing):
-dN/dt = ...
A detailed solution is given.