Answer:
3
Explanation:
The half-life of a radioactive isotope is the time it takes for the mass of the sample to halve.
This can be rewritten as follows:
[tex]m(t) = m_0 (\frac{1}{2})^n[/tex]
where
m(t) is the mass of the sample at time t
m0 is the original mass of the sample
n is the number of half-lives that passed
We see that if we take n=3, the amount of original sample left is
[tex]m(t) = m_0 (\frac{1}{2})^3 = m_0 (\frac{1}{8})[/tex]
So 3 (3 half-lives) is the correct answer.
