In a pile of steel pipes, there are 5 steel pipes in the uppermost layer and 16 steel pipes in the lowest layer. The difference between each two adjacent layers is 1 steel pipe. How many steel pipes are there in total in this pile?
一堆钢管，最上层有5根，最下层有16根，每相邻两层相差1根，这堆钢管一共有多少根．
<gadget id="calculator">5 + 15</gadget>
<output>20</output>

<gadget id="calculator">16 - 5</gadget>
<output>11</output>

<gadget id="calculator">11 + 1</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<result>120</result>