What is the perimeter of a rectangle with a length of 15 cm and a width of 5 cm?
一个长方形长15厘米，宽5厘米，它的周长是多少．
<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>