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.
