There is a section of road that needs to be repaired separately. A takes 10 days to complete, B takes 12 days to complete, and C takes 15 days to complete. Now there are A and B1 roads with the same length. A and B start repairing on A and B respectively. Started to help A repair, then turned to help B repair in the middle, and then completed the repair of two roads at the same time, may I ask: How many days did C help A repair?
有一段公路，单独修，甲需要10天完成，乙需要12天完成，丙需要15天完成，现在有A、B1条同样长的公路，甲乙分别在A、B两条路上同时开始修，丙开始帮助甲修，中途转向帮助乙修，后同时修完两条公路，请问：丙帮助甲修了几天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<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">(1/10) + (1/12) + (1/15)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / (1/4)</gadget>
<output>8</output>

<gadget id="calculator">(1/10) * 8</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">(1/5) / (1/15)</gadget>
<output>3</output>

<result>3</result>