A pile of logs is stacked into a trapezoidal cross-section, with 15 logs on the bottom layer and 5 logs on the top layer, making a total of 9 layers. How many logs are there in this pile of logs?
一堆圆木堆成横截面为梯形的形状，底层有15根，顶层有5根，共有9层．这堆圆木共有多少根圆木．
<gadget id="calculator">5 + 15</gadget>
<output>20</output>

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

<gadget id="calculator">20 * (9/2)</gadget>
<output>90</output>

<result>90</result>