In a pile of steel pipes, there are 3 steel pipes in the uppermost layer and 9 steel pipes in the lowermost layer. The difference between each two adjacent layers is 1 steel pipe. How many steel pipes are there in total?
一堆钢管，最上层有3根，最下层有9根，每相邻两层相差1根，一共有多少根．
<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</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>