What are the four main reasons some skills are in higher demand than others

The following transactions occurred during july: received $1,000 cash for services provided to a customer during july. received $4,000 cash investment from bob johnson, the owner of the business received $850 from a customer in partial payment of his account receivable which arose from sales in june. provided services to a customer on credit, $475. borrowed $7,000 from the bank by signing a promissory note. received $1,350 cash from a customer for services to be rendered next year. what was the amount of revenue for july?