To calculate the infusion rate in mL/hour, you can use the formula:

\[ \text{Infusion Rate (mL/hour)} = \frac{\text{Volume (mL)}}{\text{Time (hours)}} \]

In this case, the volume is 200 mL, and the time is 20 minutes. Convert 20 minutes to hours by dividing by 60:

\[ \text{Time (hours)} = \frac{20 \, \text{minutes}}{60} \]

Now, plug these values into the formula:

\[ \text{Infusion Rate (mL/hour)} = \frac{200 \, \text{mL}}{\frac{20 \, \text{minutes}}{60}} \]

\[ \text{Infusion Rate (mL/hour)} = \frac{200 \, \text{mL}}{\frac{1}{3}} \]

To divide by a fraction, multiply by its reciprocal:

\[ \text{Infusion Rate (mL/hour)} = 200 \, \text{mL} \times \frac{3}{1} \]

\[ \text{Infusion Rate (mL/hour)} = 600 \, \text{mL/hour} \]

So, the nurse should set the infusion pump to deliver 600 mL/hour.