Xinhua Bookstore has three types of ballpoint pens, A, B, and C. You can buy 4 ballpoint pens of type A for the price of 3 ballpoint pens of type B, and you can buy 4 ballpoint pens of type B for the price of 3 ballpoint pens of type C. It is known that the unit price of type C ballpoint pen is 3.5 yuan more expensive than that of type A ballpoint pen. What is the unit price of type A ballpoint pen?
新华书店有甲乙丙三种圆珠笔，买3支乙种圆珠笔的钱可以买4支甲种圆珠笔，买3支丙种圆珠笔的钱可以买4支乙种圆珠笔．已知丙种圆珠笔的单价比甲种圆珠笔的单价贵3.5元．求甲种圆珠笔的单价是多少元？
<result>4.5</result>