A pile of steel pipes is piled up to form an approximate trapezoid. It is known that there are 3 steel pipes in the uppermost layer and 12 steel pipes in the lowest layer. The difference between two adjacent layers is 1 steel pipe.
一堆钢管，堆成一个近似梯形，已知最上层3根，最下层有12根，相邻两层之间相差1根，这堆钢管共有多少根．
<gadget id="calculator">3 + 12</gadget>
<output>15</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<result>75</result>