Median of trade sizes

Sort first; with an even count, average the middle pair.

1. Sorted order is $3$, $5$, $8$, $12$, $15$, $20$.
2. With $6$ values the median is the average of the 3rd and 4th values.
3. That is $(8+12)÷2=10$, so the median is $10$.

Answer: $10$