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

Explanation

Let the initial mass be one unit.
Mass reamaining `=1-(7)/(8)=(1)/(8)`
A mass of `1` unit becomes `(1)/(2)` unit `1` half life `(1)/(2)` unit becomes `(1)/(4)` unit in `2^(nd)` half life
`(1)/(4)` unit becomes `(1)/(8)` unit in `3^(rd)` half life
`:.` Time taken `=3` half lifes `=3xx20=60"days"`