1. Find the concentration:
\text{Concentration of the drug} = \frac{16.6 \text{ mg}}{2 \text{ ml}} = 8.3 \text{ mg/ml}.
2. Calculate the volume required for 2.5 mg:
\text{Volume} = \frac{2.5 \text{ mg}}{8.3 \text{ mg/ml}} \approx 0.301 \text{ ml}.
Answer:
0.301 \text{ ml}