If the half life of an unstable element is 5000 years, what percentage of the parent material will be left after 10000 Years?
In this question (t½) is 5000 years , which means that after 5000 years half of the sample would have decayed and half would be left as it is. Let us start with 200g of the sample
After 5000 years ( first half life) 200 /2 = 100 g decays and 100 g remains left.
After another 5000 hours ( two half lives or 10000 years) 100 /2 = 50 g decays and 50 g remains left .
so we started with 200 g and after 10000 years 50 g of the sample will be left undecayed.
% of sample left = mass of sample left x 100 / original mass of sample
% of sample left = 50 g x 100 / 200g = 25 %