Question

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

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

Answer 1

Answer: (B)

3200 years

Answer 2

$M = M_{0}\left( \frac{1}{2} \right)^{\frac{t}{T_{1/2}}} \Rightarrow 25 = 100\left( \frac{1}{2} \right)^{\frac{t}{1600}}$⇒ t = 3200 years.