If the perimeter of a rectangle is 20 cm and the ratio of length to width is 7:3, what is the area of the rectangle in square centimeters?
一个长方形的周长是20厘米，已知长和宽的比是7：3，这个长方形的面积是多少平方厘米．
<gadget id="calculator">3 + 7</gadget>
<output>10</output>

<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">10 * (7/10) * 10 * (3/10)</gadget>
<output>21</output>

<result>21</result>