In half life the number of the sample will reduce to 50% of therir initial number.
So after first half life, sample remained will be 50%.
[tex]100\times\frac{50}{100}=50[/tex]After second half life 50% of the remaing sample will again disintegrate, so we are left with 25% of the sample.
[tex]50\times\frac{50}{100}=25[/tex]After third half life the 50% of the remaining sample will again disintegrate so we are left with only,
[tex]25\times\frac{50}{100}=12.5[/tex]So after 3 half life only 12.5% of the sample will be remaining.
