The perimeter of an isosceles trapezoid is 60 cm, the sum of the upper and lower bases is 20 cm, what is the length of each waist in cm?
一个等腰梯形的周长是60厘米，上下底的和是20厘米，每一条腰长多少厘米．
<gadget id="calculator">60 - 20</gadget>
<output>40</output>

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

<result>20</result>