If y is proportional to x, and y = 4 when x = 6, what is the constant of proportionality between them (the ratio of x to y)?

Which of the following sets of ordered pairs shows a proportional relationship?

John eats a bowl of cereal for 3 of his 4 meals each day. He finishes two gallons of milk in eight days. How much milk does John use for one bowl of cereal? (Assume he only uses the milk for his cereal.)