SOLUTION: 3. A sample of a radioactive isotope was found to be 40 mg 18 days ago and 5 mg today. a) What is the half-life of the substance? b) How many milligrams of the substance will

(a)  The ratio of the current mass to the initial mass is   =  = .


     It means that 3 (three) half-lives passed from the beginning to the current moment.


     It means that 18 days comprize 3 half-lives.  Hence, one half-life is  = 6 days.     ANSWER




(b)  12 days from today comprise  12/6 = 2 half-lives;

     So, in 12 days after today, the remaining mass will be   =  = 1.25 milligrams.    ANSWER