table is 30 yuan, and a chair is 20 yuan. Now buy 10 pieces of tables and chairs, and pay 280 yuan. How many tables did you buy?
一张桌子30元，一把椅子20元，现买桌子和椅子共10件，付款280元，桌子买了多少张．
<gadget id="calculator">30 * 10</gadget>
<output>300</output>

<gadget id="calculator">300 - 280</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">20 / 10</gadget>
<output>2</output>

<gadget id="calculator">10 - 2</gadget>
<output>8</output>

<result>8</result>