I just need to know how you find out the answers for the C rows.

Accounting for Sales Transactions Kate Company uses the perpetual inventory system. Record the journal entries for the following transactions: a. On July 16 , Kate sold $1,800 of merchandise with terms of 2/10,n/30. The cost of the merchandise was $1,080. b. On July 19 , the customer returned $300 of the merchandise from (a). The cost of the merchandise was $180. c. On July 22 , the customer paid the entire balance due to Kate