Question

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

• A. 6400 years
• B. 2400 years
• C. 3200 years
• D. 4800 years

Answer 1

The correct option is C 3200 years

To reduce 100g to 25g, it will take 2 half-lives.

$\frac{100}{2^2}=25$

Half-life = 1600 years.

Time taken = $2\times 1600$

= 3200 years.

Hence, option C is correct.