The following genuine values are from the 2011 age-structure sheet of the OCR large data set.
| Local Authority | Mean age | Median age |
|---|---|---|
| County Durham | 41.0 | 42 |
| Darlington | 40.2 | 41 |
| Gateshead | 40.5 | 40 |
| Hartlepool | 39.8 | 40 |
Calculate the mean of the four mean ages.
Calculate the median of the four median ages.
Explain why the answer to part (b) is not the median age of all residents in the four Local Authorities combined.