4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80,84,88,92,96,100,104,108,112,116,120,124,128,132,136,140,144,148,152,156,160,164,168,172,176,180,184,188,192,196 those are the multiples if 4 so their for 64 is not a multiple of 4
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.
