Let us say there are X boys, that means there are 2X girls. We know the total number of students is 60. 
So, 3X=60, giving us X=20. That means there are 40 girls and 20 boys. 
We are told that Kamal ranked 17th from the top, so 16 people ahead of him. Out of these 16, we are told that 9 are girls, meaning the remaining 7 must be boys. So, in the first 17 kids, nine girls and eight boys. The remaining boys will be behind Kamal, the number of them Is 20-8=12 boys.