Define half life of a radioactive substance.

Why is it not possible to define total life of radioactive substance?

What is meant by half life of a radioactive substance? Derive expression for them. What is relation between average life and half life?

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?

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

Distinguish between half and average life of a radioactive substance.