The perimeter of a rectangle is 20 centimeters. If both the length and width are prime numbers, what is the area of the rectangle in square centimeters?
一个长方形的周长是20厘米，如果长、宽的数都是素数，这个长方形的面积是多少平方厘米？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>