If the half-life is 1 billion years, then here's how much of the original sample remains after several periods of time:
1 billion years . . . . . 70.7%
2 billion years . . . . . 50%
3 billion years . . . . . 35.4%
4 billion years . . . . . 25%
5 billion years . . . . . 17.7%
6 billion years . . . . . 12.5%
7 billion years . . . . . 8.8%
8 billion years . . . . . 6.25%
9 billion years . . . . . 4.4%
10 billion years . . . . . 3.125%
11 billion years . . . . . 2.2%
12 billion years . . . . . 1.5625%
13 billion years . . . . . 1.1%
