if the half-life of a radioactive substance is a hundred years and how old would a rock that has 25% radioactivity left from the original amount

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After one year (one half-life), half of the radioactive atoms have decayed to form stable daughter products, and 50% of the radioactive atoms remain. After another year (two half-lives), half of the remaining radioactive atoms have decayed, and 25% of the radioactive atoms remain.

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