Seven-eighths of a sample of hydrogen-3 will have become a stable isotope after 36.9 years. What is the half-life of hydrogen-3?

If seven-eighths has become a stable isotope, one-eighth remains radioactive. The remaining fraction goes from 1 to 1/2 to 1/4 to 1/8 in three half-lives. So the half-life is 36.9 ÷ 3 = 12.3 years, choice A.

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