What is the length of a rectangle with a perimeter of 100 meters and a width of 20 meters?
长方形周长100米，宽20米，长是多少米．
<gadget id="calculator">100 / 2</gadget>
<output>50</output>

<gadget id="calculator">50 - 20</gadget>
<output>30</output>

<result>30</result>