A steel pipe factory has a pile of steel pipes, with 10 on the top and 30 on the bottom. It is known that each layer is one more than the previous layer, how many steel pipes are there in this pile.
钢管厂有一堆钢管，最上层摆10根，最下层摆30根．已知每层比上一层多1根，这堆钢管共有多少根．
<gadget id="calculator">10 + 30</gadget>
<output>40</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">21 / 2</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">40 * (21/2)</gadget>
<output>420</output>

<result>420</result>