Question

The half-life of 222Rn is approximately 4 days. A sealed vessel is found to contain 1.5 mg of 222Rn. How much radon would have been present 12 days before this measurement was taken?

Answer 1

In 12 days, 3 half-lives will have passed. Thus, the original amount will be:
1.5 x 2³
Original amount = 12 mg