The perimeter of a rectangle is 78 meters, the width is 15 meters, what is the length in meters?
一个长方形的周长是78米，宽是15米，长是多少米．
<gadget id="calculator">78 / 2</gadget>
<output>39</output>

<gadget id="calculator">39 - 15</gadget>
<output>24</output>

<result>24</result>