A salesman earns a salary of $2,500 and a commission of $325 per sale he makes. Last month, he earned a total of $6,250. Which equation could be used to find s, the number of sales he made last month?
6250 = 2500s + 325
6250 = 325(2500) + s
6250 = 325s + 2500
6250 = 325 / 2500