Al is a medical doctor who conducts his practice as a sole proprietor. During 2020, he received cash of $607,800 for medical services. Of the amount collected, $38,400 was for services provided in 2019. At the end of 2020, Al had accounts receivable of $93,100, all for services rendered in 2020. In addition, at the end of the year, Al received $17,500 as an advance payment from a health maintenance organization (HMO) for services to be rendered in 2021. a. Compute Al's gross income for 2020 using the cash basis of accounting.