Get the mean of numerical columns in a multi-index datafram : I want to calculate the mean of numerical columns in a multi-index datafram, and append the datafram with the new results as a new row.

```python
              subject  1                subject  2                          subject  3…
Country     2017    2018    Frq   2017 Score  2018 Score  2017    2018    Frq 2017 Score      2018 Score  
Argentina   12      22      100   50.  51.    12   22     100 50.  51. 
Australia   68      13      150   66.  67.    68   13     150 66.  67.  
```

I'v tried using this line, But I get a new row full with nan


```python
G20.loc['mean'] = G20.mean(axis=0, numeric_only=True)

```


Thanks