The half-life of cobalt-60 is 5.20 yr. how many milligrams of a 2.000-mg sample remains after 6.55 years?

Decay expression is stated as follows:

N (t) = N (o) e^ (-0.693t/t1/2)

N (t) = Quantity remaining after time, t; N (o) = Initial quantity, t = time, t1/2 = Half life

Therefore,

N (6.55) = 2*e^ (-0.693*6.55/5.2) = 0.835 g