Seven-eighths of a sample of hydrogen-3 will have become a stable isotope after 36 point 9 years. What is the half-life of hydrogen-3?

If seven-eighths has become stable, then one-eighth of the original hydrogen-3 remains. Each half-life halves the amount, and three halvings leave one-eighth: 1/2 × 1/2 × 1/2 = 1/8. So the half-life is 36.9 ÷ 3 = 12.3 years, choice A.

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