A rectangle has a perimeter of 20 cm and its length and width in centimeters are two prime numbers. What is the area of the rectangle?
一个长方形周长20cm，它的长和宽的厘米数是两个质数，这个长方形的面积是多少？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>