Answer:
8:12
Explanation:
Ok so the starting ratio is 4:6 
It is 20 members last year and 25 member the next year.
LAST: 20 THIS YEAR:25
When u use the ratio of 4:6 IN 25 students, u get 
8:12 LAST year. 
You can check because 8 + 12 = 20, or the total number of students.