Simple day average from 2 fact rows aggregated by order

[FinService]

Hello guys, I have a simple question about calculating average date diff (in days) between two facts of the same order.

Here is a sample data:

The result should be 4,4. 
I think I should use Aggr by Order Id but I don't know how to calculate the delta using the 2 Fact Types (SENT and DELIVERED).

Can you help with that measure? Thank you

[Brett_Bleess]

Best I have is the following, hopefully may put you on the right track:

https://community.qlik.com/t5/Qlik-Design-Blog/Set-Analysis-in-the-Aggr-function/ba-p/1463822

Regards,
Brett