The activity in ci of the elements with 12 years half-life is 14.49 ci.
We need to know about the half-life of the radioactive elements to solve this problem. The radioactive element will decay over time and follow the equation
N = No( 1 / 2 )^( t / t'' )
where N is the final quantity, No is the initial quantity, λ is the decaying constant, t is time and t'' is the half-life of a radioactive element.
From the question above, we know that
t'' = 12 years
No = 19.8 ci
t = 5.4 years
Calculate the remaining mass
N = No . ( 1 / 2 )^( t / t'' )
N = 19.8 . ( 1 / 2 )^( 5 / 12 )
N = 19.8 . 0.73
N = 14.49 ci
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