If the perimeter of a rectangle is 30 cm and the length is 8 cm, what is its width in cm?
一个长方形的周长是30厘米，长是8厘米，求它的宽是多少厘米．
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

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

<result>7</result>