Let y varialbe represent the amount of older students.
Then, x+y <= 100
Each x costs $3 thus in total 3x dollars.
Each y costs $6 thus in total 6y dollars.
The net total is, 3x+6y.
This yields two inequalities,
But there are two more, x>=0 and y>=0
These are natural inequalities.
Since you cannot have a negative amount of students going.
So we really have,
x>=0 and y>=0