Suppose 20 students delegates will attend the conference. Each school should be represented fairly in relation to it's population. How many should be selected in each school?

To find out the number coming from each school, you need to use the proportion of the school's population out of the population of all three.

Total population = 618 + 378 + 204

= 1,200 students

North Middle school:

= 618/1,200 * 20

= 10 students

Central Middle School:

= 378 / 1,200 * 20

= 6 students

South Middle School:

= 204/1,200 * 20

= 4 students