COUNT number of days since value last seen

[CMoppet]

Hello.  I have a set of data that list work orders for technicians attending breakdowns. 

I have a column called 'Serial Number', a column called 'Created Date, a column called 'Brand' and a column called 'Work Type'.  

For each row of data, I need to see if the SERIAL NUMBER already appears in the list with a WORK TYPE of either 'Repair' or 'Revisit'.  If it does, I need to find out how many days have elapsed between the two CREATED DATEs.

Once I've figured out this, I hope to be able to create a visual that shows the average number of days between breakdowns, split by the machine BRAND.

Please could someone help me understand how to calculate the first part?  I don't know how to write an expression that incorporates the fact that it should only count days between certain WORK TYPEs.

Thank you!

[lbendlin]

Please provide sample data that covers your issue or question completely, in a usable format (not as a screenshot).
https://community.fabric.microsoft.com/t5/Community-Blog/How-to-provide-sample-data-in-the-Power-BI-...
Please show the expected outcome based on the sample data you provided.

https://community.fabric.microsoft.com/t5/Desktop/How-to-Get-Your-Question-Answered-Quickly/m-p/1447...