The perimeter of a rectangle is 72 meters, the length is 21 meters, how many meters is the width?
一个长方形的周长是72米，长是21米，宽是多少米？
<gadget id="calculator">72 / 2</gadget>
<output>36</output>

<gadget id="calculator">36 - 21</gadget>
<output>15</output>

<result>15</result>