The students practice queuing. First, they line up in a solid square with 18 people on each side of the outermost layer, and then change it into a three-layer hollow square. Ask: How many people should there be in the outermost layer of the square after the modification?
学生练习排队，先正好排成一个最外层每边18人的实心方阵，后来又改为三层空心方阵，问：改完后这个方阵最外层应有几人？
<gadget id="calculator">18 / 4 / 3</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">18 * (3/2)</gadget>
<output>27</output>

<gadget id="calculator">27 + 3</gadget>
<output>30</output>

<result>30</result>