Answer:
7.
Explanation:
- The half-life is the time needed to a radioactive element to decay to its half initial concentration.
The first half-life: decay from 100 % → 50 %.
The second half-life: decay from 50 % → 25 %.
The third half-life: decay from 25 % → 12.5 %.
The fourth half-life: decay from 12.5 % → 6.25 %.
The fifth half-life: decay from 6.25 % → 3.125 %.
The sixth half-life: decay from 3.125 % → 1.5625 %.
The seventh half-life: decay from 1.5625 % → 0.78125 %.
So, the substance to decrease to below 1% of its initial level takes 7 half-lives
