Fraud Investigators Inc. operates a fraud detection service. On March 31, 10 customers were billed for detection services totaling $20,000. On October 31, a customer balance of $1,250 from a prior year was determined to be uncollectible and was written off. On December 15, a customer paid an old balance of $750, which had been written off in a prior year. On December 31, $450 of bad debts were estimated and recorded for the year. Required: Prepare journal entries for each transaction above.