Calculate the percent activity of the radioactive isotope iodine-131 remaining after 3 half-lives.

Half life of a radio-active material is refereed to the time required for reducing its activity to 50 %.

Now, when I-131 passes 1st half life, it's activity will reduce to half i. e. 50%

When, I-131 passes 2nd half life, activity of I-131 will be reduced to 25%

On passing, 3rd half-life, percent activity of the I-131 remaining will be 12.5 %.