Which equation represents an inverse variation?

Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 or more marbles, the students have a party. right now, the the jar has 24 marbles. how could mrs. morton reward the class in order for the students to have a party?

X+ y = 5 ; x - y =3 solve using linear combination

How can writing phrases as algebraic expressions you solve problems?