A trapezoid and a parallelogram have the same area and the same height. If the base of the parallelogram is 30 cm and the upper base of the trapezoid is 8 cm, what is the lower base of the trapezoid in cm?
一个梯形和一个平行四边形面积相等，高也相等．若平行四边形的底是30厘米，梯形的上底是8厘米，则梯形的下底是多少厘米．
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">60 - 8</gadget>
<output>52</output>

<result>52</result>