Mathematics teacher Li brought two iron wires of the same length, one forming a parallelogram and the other forming an isosceles trapezoid. The adjacent sides of the enclosed parallelogram are 9cm and 11cm long respectively, the waist of the enclosed isosceles trapezoid is as long as the upper base, and the lower base is twice as long as the upper base. What is the length of the base of this isosceles trapezoid?
数学李老师拿来两根同样长的铁丝，一根围成了一个平行四边形，一根围成了一个等腰梯形．围成的平行四边形的一组邻边分别长9cm和11cm，围成的等腰梯形的腰和上底同样长，下底是上底的2倍．请问这个等腰梯形的下底长多少厘米？
<gadget id="calculator">9 + 11</gadget>
<output>20</output>

<gadget id="calculator">1 + 2 + 2</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">2 * 20 * (2/5)</gadget>
<output>16</output>

<result>16</result>