Here's a simple spreadsheet in which you can practise rules of divisibility. You can enter a number (or let Excel generate one for you), then use certain shortcuts (the "rules of divisibility") to see if the number can be divided exactly by 2, 3, 4, 5 and/or 9.

Shortcuts are great, though they often make mathematics a little "mysterious". Why do these rules work? Some are easier to understand than others...