Liu Ming bought 4 copies of "Underwater World" and 2 cassettes at Xinhua Bookstore. The total cost was 58 yuan, and each cassette was 9 yuan. How much is each "Underwater World"?
刘明同学在新华书店买了4本«海底世界»和2盒磁带，一共用去58元，每盒磁带9元，每本«海底世界»多少元？
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">58 - 18</gadget>
<output>40</output>

<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<result>10</result>