The sum of the upper and lower bases of a right-angled trapezoid is 20cm, its perimeter is 35cm, and the length of one waist is twice the length of the other. What is the area of this trapezoid in cm**2.
一个直角梯形的上、下底之和是20cm，它的周长是35cm，一条腰长是另一条腰长的2倍，这个梯形的面积是多少cm**2．
<gadget id="calculator">35 - 20</gadget>
<output>15</output>

<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">15 * (1/3)</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">20 * (5/2)</gadget>
<output>50</output>

<result>50</result>