Zum the numbers and clear the board.
Select digits on the board which form the given zum number.
This is the case if the last digit of their addition is equal to the number zum.
By selecting these digits, they are removed from the board.
For each zum number that you have found you get points. For the first digit you use you will get one point. Each subsequent digit will double the number of points. So the more digits you use to form the zum number, the more points you get. Obviously you should try to achieve the highest score possible to get high in the rankings, before you clear the board.
Only the outer digits of the board can be used.