How many half-lives of the unknown radioactive substance have occurred?

Question

A 919.8ng sample of an unknown radioactive substance was placed in storages and its was measured periodically. After 47 days the amount of radioactive substance had decreased to 15.0 ng. How many half-lives of the unknown radioactive substance have occurred?

number of half-lives:? half-lives

Answer 1

For each half life the amount of radioactive material is halved, so

where n = the number of half-lives