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The half-life of radium is about 1600 years. Of 100 g of radium existing now, 25 g will remain unchanged after

1. 3200 years
2. 4800 years
3. 6400 years
4. 2400 years

Correct answer: 3200 years

Solution

The half-life of radium is 1600 years. After one half-life (1600 years), 50 g will remain. After another half-life (3200 years total), half of 50 g, i.e., 25 g, will remain. Thus, the correct answer is 3200 years.

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