As part of a physical geography coastal fieldwork investigation, students measured Pebble Long-Axis Length at four different sites along a beach transect.
| Site | Distance from Shoreline | Pebble 1 Length (mm) | Pebble 2 Length (mm) | Pebble 3 Length (mm) |
|---|---|---|---|---|
| A | 5m | 12 | 18 | 24 |
| B | 10m | 15 | 32 | 22 |
| C | 15m | 8 | 45 | 19 |
| D | 20m | 25 | 14 | 30 |
Calculate the median value for all the pebble length measurements taken during this investigation. You should show your workings.