There are 15 more students in the fourth grade of primary school in a four-layer hollow square. If an additional layer is added to the hollow part of the square, there will be 21 fewer students. How many fourth grade students are there in total?
小学四年级的同学排成一个四层空心方阵还多15人，如果在方阵的空心部分再增加一层就少21人，四年级的学生共有多少？
<gadget id="calculator">21 + 15</gadget>
<output>36</output>

<gadget id="calculator">36 + 8</gadget>
<output>44</output>

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

<gadget id="calculator">36 + 16</gadget>
<output>52</output>

<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">36 + 24</gadget>
<output>60</output>

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

<gadget id="calculator">36 + 32</gadget>
<output>68</output>

<gadget id="calculator">68 - 21</gadget>
<output>47</output>

<gadget id="calculator">36 + 44 + 52 + 60 + 47</gadget>
<output>239</output>

<result>239</result>