A certain radioactive isotope has a half-life of 15 hours. How many hours are four half-lives?

15 multiplied by 4 equals 60. so 4 half lives equals 60 hours also equals 2 full lives and one full life must equal 30 hours

Each half life takes 15 hours

starting at 0 hours, no half-lives have occurred

at 15, one half life has happened

at 30, two

follow the pattern, and you'll see that it takes 60 hours for 4 half-lives to happen