A sample of polonium-210 was left for 414 days. BA
After this time it had a mass of 1.45 x 10^-4 g
The half-life of polonium-210 is 138 days.
Calculate the initial mass of the sample

PO(1) = PO(0) / 2       1 refers to 1 half life of PO(0)

PO(2) = PO(1) / 2 = P(0) / 4        amount of PO left after 2 half-lives

PO(3) = PO(2) / 2 = PO(0) / 8     amount of PO left after 3 half-lives

414 da / 138 da = 3       3 half-lives pass in 414 da

PO(0) = 8 PO(3) = 8 * 1.45E-4 g = 1.16E-3 g = .00116 g  after 414 days