Final answer:
Barbara will have paid $540 by making $30 monthly payments over 18 months. She would then owe $2,960 at the end of the 18 months to fully pay off the $3,500 washing machine.
Step-by-step explanation:
If Barbara makes only the minimum payment of $30 per month on a washing machine costing $3,500, she will have paid 18 months x $30/month = $540 at the end of the 18 months. Since the full price of the washing machine is $3,500, after making the minimum payments for 18 months, she will owe $3,500 - $540 = $2,960 at the end of the promotional period.