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
B) 105
C) 187
D) 210
E) 280

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 would be 140 x 2 = 280 days.

The Half-Life of a Radioactive Isotope Is 140 Days