A geneticist extracts recombinant plasmid DNA from four identical 5.0 mL cultures of Escherichia coli. The yield of plasmid DNA from each culture is recorded in the table below:
| Culture sample | Yield of plasmid DNA (μ \mu\,μg) |
|---|---|
| 1 | 18.7 |
| 2 | 12.3 |
| 3 | 9.5 |
| 4 | 5.5 |
Calculate the mean yield of plasmid DNA extracted per culture sample.