State two advantages of nuclear energy. The half-life of iodine-131 is 8.0 days. A sample initially contains 640 mg of iodine-131. Calculate the mass of iodine-131 remaining after 24 days.

The half-life of iodine-131 is 8.0 days, and the total time elapsed is 24 days.

$$\text{Number of half-lives} (n) = \frac{\text{Total time}}{\text{Half-life}}$$ $$n = \frac{24 \text{ days}}{8.0 \text{ days}}$$ $$n = 3$$

The formula for remaining mass is $N = N_0 \left(\frac{1}{2}\right)^n$, where $N_0$ is the initial mass and $n$ is the number of half-lives.

$$N = 640 \text{ mg} \times \left(\frac{1}{2}\right)^3$$ $$N = 640 \text{ mg} \times \frac{1}{8}$$ $$N = 80 \text{ mg}$$

The mass of iodine-131 remaining after 24 days is $\boxed{\text{80 mg}}$.