Answer:
Carbon 14 has a half-life of about 5740 years
Since this is about 2 half-lives then 1/4 of the atoms would remain
meaning 1600 atoms were initially present , I assume that they mean the sample died 10,740 years ago
More accurately (T / 5740) = n = number of elapsed half-lives
(10740 / 5740) = 1.87 half lives
(1/2)^1.87 = .273 of original amount left
400 / .273 = 1463 atoms originally present
