Answer:
5 months
Step-by-step explanation:
Set this up as an equation, using x to represent the number of months, and solve for x
80 + 8x = 60 + 12x
20 + 8x = 12x
An easy way of solving this is putting both equations, 20 + 8x and 12x, into a graphing calculator and finding where they intersect.
x = 5
In 5 months, Xian and Kai will have the same number of stamps.