A certain class collected 648 kilograms of tree species in 18 days according to the original plan, but actually collected 18 kilograms more per day. How many days earlier than the original plan did the task be completed?
某班按原计划18天收集648千克树种，实际每天多收集18千克，实际比原计划提前多少天完成任务？
<gadget id="calculator">648 / 18</gadget>
<output>36</output>

<gadget id="calculator">36 + 18</gadget>
<output>54</output>

<gadget id="calculator">648 / 54</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>