Answer:
a. $800000
b. Account receivable Dr. 840000 Â
        To sales revenue  800000
        To sales tax payable  40000
Explanation:
a. Given the total billed amount = $840000
  Sales tax = 5%
Total revenue for the month = 840000 x (100 / 105) = $800000
b. Account receivable Dr. 840000 Â
        To sales revenue  800000
        To sales tax payable  40000