I need help on 12, 13, and 15 please can someone help me.

CP6-3 Recording Cash Sales, Credit Sales, Sales Discounts, Sales Returns, and Sales Allowances and Analyzing Gross Profit Percentage [LO 6-4, LO 6-5] [The following information applies to the questions displayed below.] Campus Stop, Inc, is a student co-op. Campus Stop uses a perpetual inventory system. The following transactions (summarized) have been selected for analysis: a. Sold merchandise for cash (cost of merchandise $143,910). b. Received merchandise returned by customers as unsatisfactory (but in perfect condition) $257,400 for cash refund(original cost of merchandise $680) c. Sold merchandise (costing $5,400) to a customer on account with terms 2/10, n/30. d. Collected half of the balance owed by the customer in (c) within the discount period. e. Granted a partial allowance relating to credit sales that the customer in (c) had not yet paid. ,630 12,000 5,880 1,640