After 160 years, the activity level of the sample will be 0.5 curies.
Half life of any radioactive waste is the half decay of that product.
The formula is [tex]\dfrac{\lambda }{\lambda_1} = 2^\dfrac{a}{b}[/tex]
Where, λ is 4 cruies
λ1 is 0.5 cruies
b = 40 years
Putting the values in the equation
[tex]\dfrac{4}{0.5} = 2^\dfrac{a}{40}\\\\\\\dfrac{a}{40} = 4\\ = 160 years[/tex]
Thus, the correct option is 160 years.
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