A rectangular desk has a perimeter of 30 decimeters, if the length is 8 decimeters, how many decimeters is the width?
一张长方形课桌，桌面的周长是30分米，如果长是8分米，宽是多少分米？
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

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

<result>7</result>