$1.$ $$7,000$ of merchandise inventory was ordered on September $2,20092.$ $$3,000$ of this merchandise was received onSeptember $5,20093.$ On September $6,2009,$ an invoice datedSeptember $4,2009,$ with terms of $3/10,$ net $30$ for $$3,250$ which included a $$250$ prepaid freight cost, was received$\mathrm{.}4.$ On September $10,2009,$ $$800$ of the merchandise was returned to the seller.$+$Based on the above information, by what date does the invoice need to be paid in order to take the advantage of the discount? September $15,2009$ September $16,2009$September $10,2009$ September $14,2009$