Trying to work out the average number between dates with others (found using a vlookup)

Hi all,

I've got a bit of a challenge I can't work out.

I have a list of dates in column A (Date Notified) and in column B a list of corresponding periods (Period Resolved), on a separate tab I have the date ranges for the periods (Start Date, End Date and Period)

I need to work out the average number of days between the Date Notified and the End date of the Period Resolved, and I cannot figure it out. If it was just two lists of dates then I could use

Formula:

=AVERAGE(Range1-Range2)

, but I need to factor in a lookup/index match I think to find out when each instance was resolved.

I've attached a sample worksheet showing the layout.

Any help would be appreciated.

Thanks

David