I'm having trouble with these few questions out of a long list of journal entries that I have to record for a project. If you could please state the journal entries for these and why that would be very helpful. Thank you!

May 2 Sold merchandise on credit to Yellow Rock Company, Invoice No. 9501, for $4,500 (cost is $2,000). I get the first part of the journal entry but don't know how to record the cost.

May 3 - Issued a $150 credit memorandum to Yellow Rock Company for defective (worthless) merchandise sold on May 2 and returned for credit.

May 12 Received a $97 credit memorandum from Jerrys Curling Supply for the return of defective office equipment received on May 10.

May 15 Cash sales for the first half of the month are $31,280 (cost is $13,200). (Cash sales are recorded daily but are recorded in a lump sum format through two entries May 15 and May 31)

May 31 Cash sales for the last half of the month are $46,025 (cost is $22,500)