Answer:
Two Half-lives
Explanation:
Let number of Parent nuclei Initially present be X,
Then, finally Parent nuclei Will remain with
daughter nuclei.
In one half- life , parent nuclei becomes half of initial.
So, starting with X parent nuclei,
After one half-life, it will degrade to .
After another half life , Parent nuclei will become half of
Which is equal to .
So, Parent nuclei have to go through Two half-lives.