The half-life of a radioactive isotope is 140 days.In how many days does the decay rate of a sample of this isotope decrease to one fourth its initial decay rate?
A)35 days
B)70 days
C)105 days
D)210 days
E)280 days

Question

Quizplus · Accepted Answer

The decay rate decreases to one fourth its initial rate after two half-lives. Since the half-life is 140 days, two half-lives is 280 days.

The Half-Life of a Radioactive Isotope Is 140 Days