The answer would be 25% remains left i.e., a radioactive substance has a half life of 5000 years, if the original sample contained 20 grams of the radioactive isotope, 25% will remains after 5000 years elapsed.
In this question (t1/2) is 5000 years , which refers that after 5000 years is half of the sample would have decayed and half would be left as it is.
Given , 20 g sample .
After 5000 years ( first half life) 20/2 = 10 g decays and 10 g remains left.
Two half lives or 5000 years 10 /2 = 5 g decays and 2.5 g remains left .
so we started with 20 g and after 5000 years 5 g of the sample will be left not decayed.
Percentage of sample left = mass of sample left x 100 / original mass of sample
Percentage of sample left = 5 g x 100 / 20g = 25 %
In conclusion, 25% remains after 5000 years have elapsed.
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