There is a project that A alone can complete in 12 days, and B alone can complete it in 15 days. After two people work together for 4 days, A is transferred away for some reason, and B works alone. How many days will it take to complete the task?
有一项工程，甲独做12天完成，乙独做15天完成．两人合做4天后，甲因故调走，由乙独做，还要几天才能完成任务？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/12) + (1/15)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 4</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) / (1/15)</gadget>
<output>6</output>

<result>6</result>