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tres1jolie
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
Student223025
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