A jar contains 9 marbles, of which 5 are blue, 3 are red, and the rest are green. What is the ratio of green marbles to red marbles?

it's equivalent, so we can say that its equal.

let's call the denominator x and the numerador is 18 less then denominator, x - 18.

multiply

2.x = 5.(x - 18)

2x = 5x - 90

90 = 5x - 2x

90 = 3x

x = 90/3

x = 30

so: