The perimeter of an isosceles trapezoid is 58 cm, the length of the lower base is 12 cm, the lower base is twice as long as the upper base, what is the waist length in cm?
一个等腰梯形周长是58厘米，下底长12厘米，下底是上底的2倍，腰长是多少厘米？
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">58 - 12 - 6</gadget>
<output>40</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<result>20</result>