bookshelf has four layers. There are 80 books on the first and second floors, and 92 books on the third floor. If you want to store 90 books on each floor in this bookshelf, how many books should be placed on the fourth floor?
一个书架有四层．第一、第二层各有80本书，第三层放92本，要使这个书架里平均每层放书90本，第四层应放书多少本？
<gadget id="calculator">90 * 4</gadget>
<output>360</output>

<gadget id="calculator">360 - 80 - 80 - 92</gadget>
<output>108</output>

<result>108</result>