What is the age of a meteorite if potassium-40 decayed from 80 g to 10 g? The half-life of potassium-40 is 1.3 billion years.

The amount went from 80 g to 10 g, which is three halvings: 80 → 40 → 20 → 10. Each half-life is 1.3 billion years, so the age is 3 × 1.3 = 3.9 billion years. The correct choice is 3.9 billion years.

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