Determine decimal fraction of C-14 remaining: 43.3 / 58.2 = 0.744 Determine how many half-lives have elapsed (1/2)n = 0.744 n log (1/2) = log (0.744) n = 0.42 Determine length of time elapsed: 5715 yr X 0.42 = 2438.16 Solution Determine decimal fraction of C-14 remaining: 43.3 / 58.2 = 0.744 Determine how many half-lives have elapsed (1/2)n = 0.744 n log (1/2) = log (0.744) n = 0.42 Determine length of time elapsed: 5715 yr X 0.42 = 2438.16.