If you were to do 50% of 30, it would be 15 students that are girls. There's 18 students that are girls though, so the extra 10% would make 18 students in the class girls. The remaining 40% would be 12 boys. So basically you're taking 10% from one side and putting it in the other because 50% of 30 is 15 (well this is at least how I would do it, I don't know how others would do it though)
