Iodine-131 is used to treat the thyroid…

Step 1. After $20\ \text{d}$. The number of half-lives is $\dfrac{20\ \text{d}}{8.02\ \text{d}} = 2.494$. The activity is $480\ \text{MBq} \times \left(\tfrac{1}{2}\right)^{2.494} = 480\ \text{MBq} \times 0.1775 = 85.2\ \text{MBq}$.

Step 2. To $30\ \text{MBq}$. The factor is $\dfrac{480}{30} = 16 = 2^4$. That is 4 half-lives, so $4 \times 8.02\ \text{d} = 32.08\ \text{d}$.