To find out how many boxes were packed per minute, you divide the total number of boxes by the total number of minutes.
In this case, the volunteers packed 50 boxes in 20 minutes:
\[ \text{Boxes per minute} = \frac{50 \text{ boxes}}{20 \text{ minutes}} = 2.5 \text{ boxes/minute} \]
So the answer is 2.5 boxes/minute.