A factory arranges 12 workers to produce a batch of parts, and it is scheduled to be completed in 15 days. After 7 days of work, 4 more workers are added. How many days can the task be completed in advance?
某工厂安排12名工人生产一批零件，预定15天完成，工作7天后，又增加4名工人．问完成任务可提前几天？
<gadget id="calculator">12 * 1 * 15</gadget>
<output>180</output>

<gadget id="calculator">12 * 1 * 7</gadget>
<output>84</output>

<gadget id="calculator">180 - 84</gadget>
<output>96</output>

<gadget id="calculator">12 + 4</gadget>
<output>16</output>

<gadget id="calculator">96 / 16 / 1</gadget>
<output>6</output>

<gadget id="calculator">7 + 6</gadget>
<output>13</output>

<gadget id="calculator">15 - 13</gadget>
<output>2</output>

<result>2</result>