The rock will be 200 years old if there have been two half-lives.
The half-life of a radioactive isotope refers to its decay period, which is the average lifetime of an atom before it disintegrates.
In this question (t½) of rock is 100 years, which means that after 100 years half of the sample would have decayed and half would be left as it is.
after two half lives or 200 years , the rock will be 200 years old.
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