A pile of steel pipes, the cross section of which is approximately trapezoidal, the top layer is 4, the bottom layer is 12 silver, and there is a difference between two adjacent layers. How many steel pipes are there in this pile?
一堆钢管，横截面近似于梯形，最上层是4根，最下层是12银，每相邻两层相差一根，这堆钢管共有几根？
<gadget id="calculator">12 + 4</gadget>
<output>16</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">16 * (9/2)</gadget>
<output>72</output>

<result>72</result>