Hey there!
The ratio is 2:1
If 1/3 of the students bring their lunch, that means 300 students do. That also means that the rest eat lunch at school, meaning 600 students do.
The ratio would be 600:300. If you simplify the ratio, it is 2:1. That means for every 2 students who buy lunch, 1 student brings lunch. Or, for every 3 students, 2 buy and 1 brings.
Good luck! Hope it helped!