Back to "management by attendance",eh?
OK, a couple ideas, but thinking more about your data model, first, is a good idea.
1. Use a relationship
You could create a relationship from that parent table to the child (portal) table on the month ID field. Then if you do a SUM from the parent side to the portal row (sum(present)), you would only see the related rows. You may need to revise your data model a bit (see my comment at top).
2. Or, do a SQL (requires no relationships set up in advance)
(pseudo code for a regular SQL statement, basic idea...):
Select sum(Present) from "PortalTableName" where parentTableNamePK = portalTableFK and ParentTableMonth=PortalTableMonth
Very much thanks for your reply but I am unable to get result on your both solutions tips.