# When Iodine-131 has a half-life of 8.0 days. How many grams of an original 160 mg sample will remain after 40 days?

Jun 3, 2018

You gots $\text{five half-lives....}$
And so we got $\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2.} . .$ with respect to the original mass....
We gots a $160 \cdot m g$ mass of ""^131I.....
And so $160 \cdot m g \times \frac{1}{2} ^ 5 = \frac{1}{32} \times 160 \cdot m g = 5 \cdot m g \ldots$