Question

A radioisotope has a five-year half life. After a given amount decays for 25 years, what fraction of the original isotope is left?

Answer 1

1/32

Explanation:

5 years from now, half is left
10 years from now, 1/4 is left
15 years from now, 1/8 is left
20 years from now 1/16 is left
25 years from now, 1/32 is left