A project is completed in 15 days by A alone, 10 days by B, and 4 days after A and B cooperate, and the rest of the work is completed by A. How many days will it take from A to the completion of the work?
一项工程甲单独做15天完成，乙10天完成，甲、乙合作4天后，剩下工作由甲完成，甲到工作完成共用多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 4</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/3) / (1/15)</gadget>
<output>5</output>

<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<result>9</result>