A pile of logs is stacked into a trapezoidal cross-section, with 12 logs on the bottom layer and 4 logs on the top layer, making a total of 9 layers. How many logs are there in this pile?
一堆圆木堆成横截面为梯形的形状，底层根数有12根，顶层根数有4根，共有9层．这堆圆木共有多少根．
<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>