Xiaoming's house is surrounded by a 15-meter-long fence against the wall to form a 3.5-meter-wide rectangular chicken pen. What is the area?
小明家用长15米的篱笆靠墙围成一个宽3.5米的长方形鸡栏，求面积是多少？
<gadget id="calculator">3.5 * 2</gadget>
<output>7</output>

<gadget id="calculator">15 - 7</gadget>
<output>8</output>

<gadget id="calculator">8 * 3.5</gadget>
<output>28</output>

<result>28</result>