A radioactive sample has an initial activity of 640 Bq640\ \mathrm{Bq}640 Bq and a half-life of 3 hours. A laboratory procedure requires it to remain in secure storage until its activity is 40 Bq40\ \mathrm{Bq}40 Bq or less. What is the minimum storage time?
151515 hours
999 hours
121212 hours
666 hours