The cross-section of a dam is trapezoidal, the upper base is 3 meters, 18 meters shorter than the lower base, and the height is half of the lower base. What is the area of the cross-section of this dam?
一个大坝的横断面是梯形，上底3米，比下底少18米，高是下底的一半，这个大坝的横断面的面积是多少．
<gadget id="calculator">3 + 18</gadget>
<output>21</output>

<gadget id="calculator">3 + 21</gadget>
<output>24</output>

<gadget id="calculator">21 / 2</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">(21/2) / 2</gadget>
<output>21/4 = around 5.25</output>

<gadget id="calculator">24 * (21/4)</gadget>
<output>126</output>

<result>126</result>