There are four cards. In each there's one of these numbers written: 1, 2, 3, 4 (every used once). Three digit numbers are made with the cards, putting the cars variously. How many three digit numbers can be made up, that can be divided by three?
My first question: is there a formula that'll help me to calculate this, or do I need to write all the possible three digit numbers and choose ones that can be divided by 3?