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→+424→+4…
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