In the given question it is already given that the population of Centerville is 9000. It is also given that the population of North Fork is 18/25 the of the total population of Centerville. With these vital information's the solution to the problem can be easily found.
Population of Centerville = 9000
North Fork's population = 9000 * (18/25)
= 162000/25
= 6480
So the population of North Fork is 6480. Hope this has helped you to learn the way this type of problems can be solved.