need help making periodic inventory system

In Class Exercise: Purchases Using the Periodic Inventory System Serrano Company On September 1, 2013, Serrano Company purchased 70 units of Product A for $35,000 cash and also paid $1,500 transportation costs related to this purchase. On the same date, Serrano Company purchased 100 units of Product B for $10,000 on credit; however, the seller paid the $1,200 freight. The credit terms for Product B were 2/10, n/30. On September 3rd, Serrano Company determined that 5 units of Product A were defective, so they were returned to the seller. Serrano Company paid for its purchase of Product B on September 9th. On September 10th, Serrano Company purchased 90 units of Product C for $8,000 on credit with terms 1/10, n/30. The seller paid the freight. Serrano Company paid for its purchase of Product C on September 21st