There is a pile of logs piled up into a trapezoidal cross section, with 3 logs on the top layer, 7 logs on the bottom layer, and 1 log more on the next layer than the previous layer. A total of 5 layers are piled up. How many logs are there in this pile?
有一堆圆木堆成横截面近似梯形状，最上面一层有3根，最下面一层有7根，下一层比上一层多1根．一共堆了5层，这堆圆木共有多少根．
<gadget id="calculator">3 + 7</gadget>
<output>10</output>

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

<gadget id="calculator">10 * (5/2)</gadget>
<output>25</output>

<result>25</result>