The half-life of radioactive isotope X is 20 yr. It decays to another element Y which is stable.The two elements X and Y were found to be in the ratio 1:& in a sample of given rock is estimated to be

40 yr

60 yr

80 yr

100 yr

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Solution

60 yr

As space straight N over straight N subscript straight o space equals space open parentheses 1 half close parentheses to the power of straight n straight N over straight N subscript straight o space equals space open parentheses 1 half close parentheses cubed space equals space 1 over 8 Number space of space half space lives space equals space 3 rightwards double arrow space straight T space equals space 20 space yr therefore space straight T space equals space straight t over straight n straight t equals space straight T space straight x space straight n space equals space 20 space straight x space 3 space yr space equals space 60 space yr

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