I need to retrieve an average TimeSpan from a List of grouped objects, but have no idea where to start. 

Each object in the list has a property of type TimeSpan and I've grouped the List by another property. Now I need the average TimeSpan from each group.

```
List<Item> Items = new List<Item>();
Item _item1 = new Item { Category = "A", Duration = _someTimeSpan1};
Item _item2 = new Item { Category = "B", Duration = _someTimeSpan2};
Item _item3 = new Item { Category = "A", Duration = _someTimeSpan3};
Items.Add(_item1);
Items.Add(_item2);
Items.Add(_item3);

var _groupedItems = Items.GroupBy(i => i.Category);

```

In the above example \_item1 and \_item3 are grouped on Category and I need the average of Duration for those two items that are in the group.

Any help is appreciated.