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