We can solve this problem by setting up an equation. In the beginning, Xian and Kai has got respectively 56 and 80 stamps. Let's denote the number of the month, when they will have the same amount of cards with an x. Then after x months, Xian and Kai will respectively have 56 + 12x and 80 + 8x cards.
By setting up an equation, we can write that:
56 + 12x = 80 + 8x
4x = 24
x = 6
The correct answer is after six months they will have the same amount of cards. After 6 months, Xian and Kai will each have 128 cards.