12 analyze what mass of radioactive isotope
will be left after three half-lives?

1/8 of original sample.

Explanation:

Consider your final amount is 100 g.

After three half lives:

at 0 = 100 g

1 half life = 100/2  = 50 g

2 half life = 50/2 = 25 g

3 half life = 25/2 = 12.5 g

Know multiply the amount after three half lives with 8.

12.5 g × 8 = 100 g

So, after three half lives 1/8 of original sample is present.