Each copy of "Anime World" is 10 yuan, and each copy of "Universe World" is 20 yuan. Teacher Li bought 7 books with 100 yuan. How many copies of "Universe World" did he buy?
每本《动漫世界》10元，每本《宇宙世界》20元，李老师用100元钱买了7本书，《宇宙世界》买了多少本．
<gadget id="calculator">20 * 7</gadget>
<output>140</output>

<gadget id="calculator">140 - 100</gadget>
<output>40</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

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

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<result>3</result>