What percentage of a sample original radioactive remains after 5 half-lives?

3.125% If at first there is 100 atoms after 1 half lives there is 100/2=50 atoms after 2 half lives there is 50/2=25 atoms after 3 half lives there is 25/2=12.5 atoms after 4 half lives there is 12.5/2=6.25 atoms after 5 half lives there is 6.25/2=3.125 atoms So there is 3.125% after 5 half lives.