For a project, it takes 18 days for A to work alone, 15 days for B to do it alone, and after 6 days of cooperation between the two, B will do the rest alone. How many days will it take to complete?
一项工程，甲单独干要18天，乙单独干要15天，二人合作6天后，剩余的由乙单独干，还要几天可以完成？
<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

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

<gadget id="calculator">(1/18) + (1/15)</gadget>
<output>11/90 = around 0.122222</output>

<gadget id="calculator">(11/90) * 6</gadget>
<output>11/15 = around 0.733333</output>

<gadget id="calculator">1 - (11/15)</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">(4/15) / (1/15)</gadget>
<output>4</output>

<result>4</result>