How much radon would remain in the tube?

Question

The half-life or radon-222 is approximatley 4 days. If a tube containing 1.00 microgram of radon were stored in a hospital clinic for 12 days, how much radon would remain in the tube?

Answer 1

12 days is 3 half-lives. Therefore the amount of material remaining is:

1.00 * 1/2^3 = 1.8 microgram