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12.5%. The half life of an element is the time it takes for half of the nuclei in any sample to decay.

so after 5 million years, theres 50% left, after another 5 years, theres only 25% left, and after another 5 years, theres 12.5% left

a) 2 to the minus 4 is a million/sixteen, 6.25% is a million/sixteen after one 0.5 life you have 50% of unique amt, after yet another you're at 25%, after a third you're at 12.5%, after the 4th you're at 6.25%