Hi Franscisco,
I just finished setting something similar to your scenario up in my Account, I got this to work, so I will share my details.
First, here is a screenshot of my screen of the successful update after pushing the LoopGrid button, the Res field in the grid was blank before:
Here is a screenshot from the Screen Designer of my Action Set for the LoopGrid button, note the 'arRes' action result was created outside the loop, all I did was add it here, but used the "Type In" option, then left the value blank.
Next, is a screenshot of the "Retrieve Value from Table" action for one of the AR_Num values, just do the same action for the other field(s):
Do your Math calculation next, take AR_Num1 and ADD it with AR_Num2, and save it to the Action Result Name of the 1st Action Result you created outside your loop, in my case 'arRes'.
Once you have this arRes value, the last step in the loop would be to "Update Field in Table", add this value to the 'res' field in your grid, make sure to Update this where RecordID = Value From Row RecordID.
Note, if you don't create the empty action result before your loop, then you will only be able to update the last value in your grid. Please let me know if this helped, or if you have any more questions.
-Ben