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Question:

Half life of a radioactive element is 8 years. How much amount will be present after 32 years?

1/16

1/4

1/8

1/32

Solution:

After n half-lives, the fraction of radioactive element present is given by (1/2)^n. For 4 half-lives (32 years = 4 times 8 years = 4 half-lives), the fraction is (1/2)^4 = 1/16