Radioactive decay is a pseudo-first order reaction. When you know the half-life of the material, you could use this equation.
A= A₀(1/2)^t/h
where
A is the final activity
A₀ is the initial activity
t is the time
h is the half-life
A = (0.64)(1/2)^88/44 = 0.16 mbq
