You can change a valid SUM at any stage – this may be tactical or because you are stuck. If this is done and creates a new valid SUM you can shout “SUM”. As long as each line or column is a valid SUM then almost anything goes!
The winner of the game is the first person to use all of their tiles up to create valid SUMs. Upon doing so the player should shout “SUM FUN”. At this stage the calculations are checked to confirm the SUMs are correct.
If at any stage no one can make a SUM, all players can agree to take another tile from the pile. Each player can choose whether to pick a number or symbol tile.
If a player declares “SUM FUN” and, upon checking, there are invalid SUMs, that player is disqualified from the game and all of their tiles are returned to the pile. The game then continues as normal.
If, at the end of the game, nobody can use their tiles and a stalemate is agreed, the player with the fewest tiles left is the winner. If still level, each player involved must add their remaining number tiles together, with the lowest total declared the winner. (Note there are no penalties for being left with symbols tiles).