The blackboard in the classroom is rectangular, the perimeter is 40 meters, and the ratio of length to width is 13:7. What is the area of this blackboard in square meters?
教室黑板为长方形，周长是40米，长与宽之比13：7，这块黑板的面积是多少平方米．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">13 + 7</gadget>
<output>20</output>

<gadget id="calculator">13 / 20</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">20 * (13/20)</gadget>
<output>13</output>

<gadget id="calculator">7 / 20</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">20 * (13/20) * 20 * (7/20)</gadget>
<output>91</output>

<result>91</result>