project can be completed by team A alone in 15 days and by team B in 12 days alone. Now after 4 days of cooperation between A and B, because team B has asked for sick leave, team A can only do the rest of the project, so how many days will it take for team A to complete the project?
一项工程，甲队单独做15天完成，乙队单独做12天完成．现在甲、乙合作4天后，由于乙队请了病假，剩下的工程只能由甲队单独来做，那么甲队还需要几天才能完成这项工程？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/15) + (1/12)</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>