Writing and Using Explicit Rules for Arithmetic Sequences

Let $A(n)$ be the sequence representing the number of tires in the bin. After the first purchase, the bin has $20$ tires. This means that our first term, $A(1),$ is $20.$ For each new customer, the bin gains another $4$ tires which makes the common difference of the sequence $4.$ $$20\stackrel{+4}{\to }24\stackrel{+4}{\to }\dots$$

Finding our formula

The generic form of an explicit formula of an arithmetic sequence is $$A(n)=A(1)+(n-1)d,$$ where $A(1)$ is the initial value and $d$ is the common difference. By substituting $A(1)=20$ and $d=4$ into the above formula, we get our equation. $$A(n)=20+(n-1)4$$