What was the cost of goods sold?

Use the question to answer this question:

Opening stock	₦
Department A B	100 800
Purchases: A B Wages of workers Salaries	1500 2000 50 100
Sales A B	3000 5000

Expenses are to be shared in the ratio of sales.

Cost of goods sold = Opening stock + Purchases – closing stocks
20,000 + 70,000 – 15,000
⇒ ₦75,000

The exchange of goods for goods in foreign trade is known as?