Answer:
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
