If there are a number of students and they are arranged in a three-layer hollow square, then there will be 9 more people, and if the hollow part is added with two layers, the number will be 15 fewer. Then how many students are there.
有学生若干人，列成三层中空方阵，那么就多9人，中空部份增列两层，则少15人，那么有学生多少人．
<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<gadget id="calculator">9 + 7</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 8</gadget>
<output>16</output>

<gadget id="calculator">16 / 8</gadget>
<output>2</output>

<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">24 - 4</gadget>
<output>20</output>

<gadget id="calculator">6 + 2</gadget>
<output>8</output>

<gadget id="calculator">8 * 4</gadget>
<output>32</output>

<gadget id="calculator">32 - 4</gadget>
<output>28</output>

<gadget id="calculator">8 + 2</gadget>
<output>10</output>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<gadget id="calculator">40 - 4 - 15</gadget>
<output>21</output>

<gadget id="calculator">4 + 12 + 20 + 28 + 21</gadget>
<output>85</output>

<result>85</result>