Answer:
28650 yrs / (5730 yrs / hlf-lif) = 5 half-lives
(1/2)^5 = 32
Since 1/2 of the sample decays every half=life after 28650 yrs only 1/32 of the original sample remains
2E12 = 2000E9 2000 billion atoms were originally present
2000 / 32 = 62.5 billion atoms present after 5 half-lves
2) 4.25E9 / 8.50E9 = 1/2
1/2 of the original sample remains
Since the half-life is 5730 yrs one would estimate the age of the sample to be 5730 years