For the half-life of a certain substance, the equation that would best represent the scenario is,
At = Ao(0.5^t/n)
where At is the amount at any time t, Ao is the original amount, and n is half-life. Substituting the known values,
At = (220 mg)(0.5^(396/72)
At = 4.86 mg
Thus, after 396 days, there will only be 4.86 mg of Terbium-160.
