Similar to placing letters in Scrabble, teams take alternate turns placing 1-8 tiles to build a valid sum in horizontal or vertical direction. In each sum you must use at least 1 block which is already on table.
For each digit in a sum you get its value as victory points. So you try to use high numbers which are already placed. You can place high numbers from your own blocks too, but your opponents can profit when you place them easily available and combinable with other placed blocks. You should also try to hold good combinations of digits and operators for later turns.
While it requires calculating, it is more a puzzle game than a math game. You mainly search for possibilities by using deduction logic instead of calculating (which is induction logic).