January $5$: Purchased $100$ units of inventory on account for $$20$ each, terms $2/10,$ n$/30.$

January $8$: Returned $10$ units of inventory from the January $5$ purchase due to defects.

January $12$: Sold $60$ units of inventory on account for $$40$ each, terms $1/15,$ n$/45.$

January $14$: Received payment for the January $12$ sale within the discount period.

January $18$: Paid the supplier for the inventory purchased on January $5$ within the discount period.