The perimeter of a rectangle is 20 decimeters, the width is 3.5 decimeters, what is the length in decimeters
一个长方形的周长是20分米，宽是3.5分米，长是多少分米
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 3.5</gadget>
<output>6.5</output>

<result>6.5</result>