Calculate time in hours between problem raised and fixed where multiple criteria are true

Hi all,

I have attached some mocked up data to help illustrate the problem.

B:B must be the same Cust_Id
C:C must be the same Prob_Desc
D:D must be 'BLANK'
E:E must be 'NOT BLANK'
When all above are TRUE Calculate hours between Problem being logged and finished.

Constraints;
Same Cust_Id may appear again with different Prob_Desc.
Same Prob_Desc may appear with different Cust_Id.
Same Cust_Id may appear again in the future with the same Prob_Desc.

Can't get my head around it. I can get bits of it to work, but never at the same time.

Mike