A rectangular vegetable field has a perimeter of 100 meters and a length of 30 meters. How many meters is the width of this vegetable field?
一个长方形菜地的周长是100米，长是30米．这个菜地的宽是多少米？
<gadget id="calculator">100 / 2</gadget>
<output>50</output>

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

<result>20</result>