A technician measures the activity of a sample of a radioactive tracer used in an industrial process. The activity is 720 Bq720\ \mathrm{Bq}720 Bq when the sample is first measured and 90 Bq90\ \mathrm{Bq}90 Bq when it is measured again 12 hours later. The activity changes only because of radioactive decay.
What is the half-life of the tracer?
1.51.51.5 hours
444 hours
666 hours
333 hours