Assuming that Xian starts with 0 stamps, you set the equations equal to each other to find what month they have the same amount of stamps.
(68 from Kai's original stamps) (8 from how many per month)
(0 from Xian's original stamps) (12 from how many per month)
68+8m=0+12m
solve for m in this equation and you get 68=4m
and 68/4 = 17 months
For how many stamps each will have, you plug in 17 into m in Xian's equation (you could do either but Xian's is easier):
0+12m
0+12(17)=204
204 stamps.
Hope this helped, it was kinda long lol
