The table below shows the annual plastic waste discharge into the ocean from selected coastal river estuaries in Southeast Asia.
| River Estuary | Catchment Population (thousands) | Annual Plastic Discharge (tonnes/year) |
|---|---|---|
| Pasig River | 12,500 | 38,200 |
| Ciliwung River | 8,200 | 25,100 |
| Mekong Delta (distributary) | 15,400 | 41,700 |
| Solo River | 6,100 | 18,300 |
| Irrawaddy River | 18,900 | 52,600 |
| Chao Phraya | 11,300 | 33,500 |
Calculate the mean value for annual plastic discharge from the data shown. You must show your working out.