Wang Ming went to buy stationery. He found that the average price of compasses and fountain pens was 9.20 yuan. If the unit price of one kind of notebook was added, the total price of the three kinds of stationery was 31.80 yuan. How much is the unit price of the notebook that Wang Ming wants to buy?
王明去购买文具，他发现圆规和钢笔的平均价是9.20元，如果加进一种笔记本的单价后，三种文具的总价是31.80元．王明要购买的这种笔记本单价是多少元？
<gadget id="calculator">9.2 * 2</gadget>
<output>18.4</output>

<gadget id="calculator">31.8 - 18.4</gadget>
<output>13.4</output>

<result>13.4</result>