The graph below shows the half life values of parent isotopes Based on the graph, it can be concluded that the isotope which is most likely to be found with 12.5% of its original amount in 42 billion years is

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.

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