List missing numbers in range (not as an array forumla)

Hi AliGW, I am using 365 (have just updated my profile to reflect)

Another option

Formula:

Please Login or Register  to view this content.

Thanks for that & the Rep.

All solutions worked perfectl. Thank you all very much.

Rep added for all and thread marked assolved

You're welcome & thanks for the feedback.

If efficiency is what you want you could do it with Power Query.

Build a table from "Production Task #", another one from "Production Line1 and Production Line 2".

Add them both to power query in the data tab, "From table or range".

The in power query editor you will see 2 queries. The one from "Production Task #" which you'll leave as is. And another from the Production line 1 and 2 table.
Right click this second query and duplicate.
Then on the first remove the second column (p. line 2). And rename the remaining column to "Task". And rename the query to "ProductionLine1" (for example).
Then on the duplicated query remove the first column (p. line 1), and rename the remaining column also to "Task". And rename the query to "ProductionLine2".
The select the "ProductionLine1" query and go to the menu "Combine/append queries to create a new one" (from the dropdown menu).
Select ProductionLine1 and ProductionLine2.
A new query will be created which you can rename to "AttendedTasks" for example. This list contains all the tasks done by p.line 1 and 2.
Then select the first query "ProductionTask#" and go to menu "Merge Queries to create a new one".
In the query selection window you leave "ProductionTask#" in the first and select "AttendedTasks" on the second. The you have to click the column header of both example data to tell power query which column it should use to make the join. As you have only one column in each table there isn't much choice.
Then in Join Kind select "Left Anti (rows only in first)" (second option from the bottom), and click ok.
This new query will have all the numbers you need, which are in Production Tasks but not in p.line1 or 2.
Rename the first column to "Production Line 3".
After this go to Close & Load.
One new page will be created for eath query by default. Just delete the pages you don't need.
You can move or cut and paste the result query (Production line 3) table where you need it.

I just noticed that you where missing the task number 19 in your array formula solution.

Good thing about having data in tables, is that as you add new data al queries are updated, you just have to click "Update All" in Data tab.

One more formula for the mix...

=SORT(UNIQUE(VSTACK(B3:B22,TOCOL(D3:E12,1)),,1))