The half-life of`""_(38)^(90)`Sr is 28 years. What is the disintegration rate of 15 mg of this isotope?

The half-life of ""^(232)Th is 14.05 billion years. Calculate its average life.

The half-life of Radon is 3.8 days. Define half-life.

The half-life period of ""_(53)I^(125) is 60 days. What percent of radioactivity would be present after 240 days ?