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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 day

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Answer:

It mus be 70 because you half 140 hence its called 'half life'

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