The lower base of a trapezoid is 3 times the upper base. If the upper base is extended by 24 cm, it becomes a parallelogram. What is the upper base of the trapezoid in cm?
一个梯形下底是上底的3倍，如果把上底延长24厘米，就变成了一个平行四边形，梯形的上底是多少厘米．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<result>12</result>