The half-life of a radiaoactive substance is 100 years. In how many years its activity will deacy to 10% of its initial value?

The half-life of a radioactive substance is 1,672 years. If the initial mass of the substance is 1 g, after how many years will only 1 mg of it to be left behind?

A radioactive isotope has a half-life of T years. How long will it take the activity to reduce to - 1% of its original value?