The upper base of a trapezoid is 4 cm, the lower base is 3 times the upper base, and the height is 5 cm. If the lower base of the trapezoid is shortened to form a parallelogram with a base of 4 cm, how much square centimeters will the area be reduced?
一个梯形的上底是4厘米，下底是上底的3倍，高是5厘米，如果把这个梯形的下底缩短，成为一个底边为4厘米的平行四边形，面积减少多少平方厘米．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

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

<gadget id="calculator">8 * (5/2)</gadget>
<output>20</output>

<result>20</result>