xian = 56 stamps + 12 per month : 12m + 56
kai = 80 stamps + 8 per month : 8m + 80
when will they have same number of stamps ? set them equal.
m = number of months
12m + 56 = 8m + 80
12m - 8m = 80 - 56
4m = 24
m = 24/4
m = 6.....at 6 months, they will have the same number of stamps.
12m + 56 = 12(6) + 56 = 128
8m + 80 = 8(6) + 80 = 128
They will both have 128 stamps