Answer:
Thorium-235
Explanation:
Half-life is defined as the time taken for a radioactive material to reduce to half of its original amount. If the original amount of a radioactive substance N is 100%, then;
1st half-life- 50% of N is left
2nd half life - 25% of N is left
3rd half-life- 12.5% of N is left
The half-life of Thorium-235 is 15 billion years, hence three half lives will take place in 45 billion years. Hence 12.5% of the original amount of Thorium-235 present will remain after about 42 billion years.