In a pile of steel pipes, each upper layer has one less than the lower layer, and the uppermost layer has 3 steel pipes. There are 7 layers in total. How many steel pipes are there in this pile?
一堆钢管，每上面一层比下面一层少一根，最上层是3根，共有7层，这堆钢管一共有多少根？
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">3 + 3 + 6</gadget>
<output>12</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">12 * (7/2)</gadget>
<output>42</output>

<result>42</result>