Terbium-160 has a half-life of about 72 days.

after 396 days, about how many miligrams of a 220 mg sample will remain.

a) 5mg
b)12mg
c)40mg
d)194mg

show work or give the equation i am so confused

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. 

The correct answer is d, hope this