Journal entries for Periodic and Perpetual inventory!!!?

Periodic Inventory System:

(Dr) Cash / Accounts Receivable $12000

(Cr) Sales $12000

Note: No journal entry is made to effect the inventory balance. Inventory sold will be incorporated under the cost of sales only after year - end count. If it was sold on Cash, then debit cash. If on account or charge, then debit Accounts Receivable.

Perpetual Inventory System:

(Dr) Cash / Accounts Receivable $12000

(Cr) Sales $12000

(Dr) Cost of Sales $8000

(Cr) Merchandise Inventory $8000

Note that under the Perpetual Inventory System, an entry is made to effect the sales (deduction of inventory balance) of inventory. If it was sold on Cash, then debit cash. If on account or charge, then debit Accounts Receivable.

I have a accounting project that I have to do, and any help would be appreciated? It's journal entries using perpetual inventory. Here is a couple of the problems that I am having issues having to figure out. Issued a $175 credit memorandum to Knott Co. for defective (worthless) merchandise sold on Dec. 28 and returned for credit. The total selling price (gross) was $4,275. Received a $798 credit memorandum from Parker Products for the return of merchandise purchased on Dec 29

Perpetual - book all purchases to Purchases. Periodic, book them to COGS.

Perpetual --

Cash $12000

Sales Revenue $12000

COGS $8000

Merchandise Inventory $8000

Periodic -- cash/sales revenue stays the same

Merchandise Inventory $8000

Purchases $8000