Answer:
- North Middle School = 10 students 
- Central Middle School = 6 students 
- South Middle School = 4 students
Explanation:
First calculate the total number of students in all the schools:
= 618 + 378 + 204 
= 1,200 students
Use this number to calculate the proportion of the total population that a school so that this can then be used to determine the proportion of the 20 delegates they should sent. 
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