A radioactive substance having N nuclei has activity A. Write down an expression for its half life in terms of A and N?

A radioactive substance contains 10^(15) atoms and has an activity of 6.5 xx 10^(11) Bq. What is its half life.

The half - life period of a radioactive substance is 20 days. What is the time taken for 7//8^(th) of its original mass to disintegrate ?