Business Finance Homework Help

I’m trying to learn for my Accounting class and I’m stuck. Can you help?

Balances at 31 January 2009:
Debtors control account………………………..$32,400
Creditors control account………………………$25,200
Inventory……………………………………………$30,000

Balances at 28 February 2009:
Debtors control account………………………..$24,000
Creditors control account………………………$29,160
Inventory……………………………………………$36,000

Extract from cash payments journal at 28 February 2009:
Payments to creditors……………………………$41,040

Additional Information:
a) Gross profit mark-up is 20% on cost
b) Inventory is kept on the perpetual system
c) All purchases and 80% of sales are on credit

Required:
a) Calculate purchases.
b) If purchases during February were $39,000 what is the Cost of Goods Sold for February?