chem help plz?

Question

cobalt-60 is a beta emitter with a half-live of 5.3 years. Approximately what fraction of cobalt-60 atoms will remain in a particular sample after 26.5 years?

1. 1/5
2. 1/16
3. 1/26
4. 1/32
5. 1/64

Answer 1

5. 1/64

Half life is 5.3 so it went through 5 divisions by 2. 2 to the fifth power is 64.

Answer 2

If half-life = 5.3 years, in 26. 5 years it will half itself 5 times so

(1/2)(1/2)(1/2)(1/2)(1/2) = 1/ 32