What is the term for the time it takes for half of a radioactive substance to decay?

If a substance has a half-life of 10 years, how much of a 100 gram sample will remain after 30 years?

Which of the following best describes the half-life of a radioactive substance?

If you start with 256 grams of a radioactive isotope and after 6 hours you have 16 grams left, what is the half-life of the isotope?

The half-life of carbon-14 is approximately 5730 years. If you start with a 10 gram sample, how much carbon-14 will remain after 17190 years?

A radioactive substance has a half-life of 4 years. If the initial amount is 100 grams, how long will it take for the substance to decay to 25 grams?