A rule is the relationship between two quantities. For example, a student's weekly allowance is $10$ USD. The relation between the amount of money $y$ after $x$ weeks can be expressed as: $y=10x.$ For example, the rule can be used to calculate the amount of money the student earned after $4$ weeks, $10\cdot {\color{#0000FF}{4}} = 40 \text{ USD.}$