A pile of steel pipes, with 7 at the top and 13 at the bottom, with 1 more per floor from top to bottom. How many steel pipes are there in total?
一堆钢管，最上层7根，最下层13根，从上到下每层多1根．这堆钢管一共有多少根？
<gadget id="calculator">7 + 13</gadget>
<output>20</output>

<gadget id="calculator">13 - 7</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">20 * (7/2)</gadget>
<output>70</output>

<result>70</result>