Explanation on this half life question

Question

A 10 half-life storage period is recommended for materials containing Cs-137. Utility companies are required to store materials containing Cs-137 for 25 years. Does this storage period satisfy the 10 half-life recommendation or do these materials need to be stored for a longer period of time? Explain.

Answer 1

The half-life of Cs-137 is about 30 years, so 10 half-lives would be 300 years. You can see that 25 years is not even one half-life.