Mathematical watercolor pen
Last question: Because there are three more watercolor pens 58.5-40.5= 18 yuan, 18 is the money for three watercolor pens 18/3=6 yuan and one watercolor pen 6 yuan. 58.5-6*6=22.5 yuan. A: A pen is 22.5 yuan, and a watercolor pen is 6 yuan.