A project can be completed by A in 12 days and by B in 15 days. After A and B cooperate for 4 days, A has another task, and B will complete the remaining work alone. How many days will it take for B to complete the entire project?
一项工程，甲独做12天完成，乙独做15天完成．甲乙合作4天后，甲另有任务，剩下的工作由乙单独完成，乙还要几天完成全部工程？
<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>