A project can be completed in 15 days by A alone, and both B and C can complete it in 10 days alone. Now the three are working together, and A left on the way because of something. It is known that the project took 4 days to complete. How many days did you ask A to do?
一项工程，甲独做15天完成，乙和丙独做都要10天完成．现在三人合做，途中甲因事离开，已知这项工程共用了4天完成．求甲做了多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) + (1/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) * 4</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

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

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

<result>3</result>