Answer:
There are multiple possibilities. One is there are 12 boys, and 16 girls. 12+16= 28 which is between 20 and 30. That would be 12:16, which can be simplified to 3:4. There is also 9 boys, and 12 girls. that would be 9:12, which can be simplified to 3:4. Also, 9 + 12 = 21, which is between 20 and 30.