A jewelry maker works 6 days a week, makes 15 necklaces per day on average, and charges $18 a necklace. If she lowers her price to $10 a necklace, by how many necklaces per day on average must she increase her production to make the same amount of money per 6-day workweek?

6 days a week ... 15 necklaces per day ... 18 per necklace

6 * 15 * 18 = 1620 for a 6 day workweek

6 days a week ... x necklaces per day ... 10 per necklace ... makes 1620

10x = 1620

x = 1620/10

x = 162 ... 162 necklaces for a 6 day work week

162/6 = 27 ... she should make 27 per day ... so she should increase by (27 - 15) = 12 per day