A rectangular yard is surrounded by a 20-meter-long fence. It is known that the length is 7 meters. How many square meters does the yard occupy?
用20米长的篱笆围成一个长方形院子，已知长是7米，院子的占地面积是多少平方米？
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 7</gadget>
<output>3</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>