There are 36 tables and chairs in the classroom. The ratio of tables to chairs is 7:2. Later, some tables were bought, accounting for 80% of the total. How many tables were bought?
教室有桌子和椅子共36张，桌子和椅子的比是7：2，之后又买来了一些桌子，桌子数占总数的80%，又买来多少张桌子？
<gadget id="calculator">7 + 2</gadget>
<output>9</output>

<gadget id="calculator">2 / 9</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(2/9) / (1/5)</gadget>
<output>10/9 = around 1.111111</output>

<gadget id="calculator">36 * (10/9)</gadget>
<output>40</output>

<gadget id="calculator">40 - 36</gadget>
<output>4</output>

<result>4</result>