Magician's Hat

Stage: 3 Short Challenge Level: Challenge Level:1
Kan must know for sure that Roo's cards are either both even or both odd. The only way he can be sure of this is if he's selected the three even cards leaving Roo only odd numbered cards. Therefore Kan has cards $2$, $4$ and $6$ and the sum of his cards is $12$.

This problem is taken from the UKMT Mathematical Challenges.