You will be given a definition of a task first, then some input of the task.
The task is to generate the correct answer for a given math problem.

Problem: village x has a population of 72000 , which is decreasing at the rate of 1200 per year . village y has a population of 42000 , which is increasing at the rate of 800 per year . in how many years will the population of the two villages be equal ?
Output:
15