There are 180 students in the third grade of a certain school who are arranged in a three-layer hollow square. How many people are there on each side of the outer layer of the square?
某学校三年级同学180人，排成一个三层空心方阵，这个方阵外层每边多少人？
<gadget id="calculator">180 / 4 / 3</gadget>
<output>15</output>

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<result>18</result>