Question

What proportion of 235U was present in a rock formed 3,000×106 years ago, given that the present proportion of 235U to 238U is 140?

Answer 1

Let the proportion of 235U and 238U at t = 0 be x : (1 − x). Present day radioactive atoms for the two components after time t will be

Substituting these values in (3) and solving for x we get x = 1/22

Therefore, the ratio of 235U and 238U atoms 3 × 109 years ago was 1/22:21/22 or 1:21