If the length of a rectangle is 15 cm and the width is 4 cm, what is its perimeter?
一个长方形的长是15厘米，宽是4厘米，它的周长是多少．
<gadget id="calculator">15 + 4</gadget>
<output>19</output>

<gadget id="calculator">19 * 2</gadget>
<output>38</output>

<result>38</result>