Xiao Ming uses rectangular pieces of cardboard with a length of 15 cm and a width of 12 cm to form a square, at least how many pieces of cardboard are used.
小明用长15厘米，宽12厘米的长方形硬纸片拼成正方形，至少用多少张硬纸片．
<gadget id="calculator">2 * 2 * 3 * 5</gadget>
<output>60</output>

<gadget id="calculator">60 / 12</gadget>
<output>5</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>