There is a pile of steel pipes on the construction site. The cross section is a trapezoid. It is known that there are 4 steel pipes on the top layer and 12 steel pipes on the bottom layer. There is a difference of one steel pipe between each two layers. There are 9 layers in total. How many steel pipes are there in this pile?
工地上有一堆钢管，横截面是一个梯形，已知最上面一层有4根，最下面一层有12根，每两层相差一根，共堆了9层，这堆钢管共有多少根．
<gadget id="calculator">4 + 12</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>