In a classroom, if A takes 10 minutes to clean, B takes 12 minutes to clean, and C takes 15 minutes to clean. There are the same two classrooms A and B. A is in classroom A, and B starts cleaning in classroom B at the same time. C helps A to clean first, and then helps B to clean the classroom in the middle. Finally, the two classrooms are cleaned at the same time. How long did C help A to clean? (The time it takes for C to go to B classroom in the middle is ignored)
一间教室如果让甲打扫需10分钟，乙打扫需12分钟，丙打扫需15分钟．有同样的两间教室A和B．甲在A教室，乙在B教室同时开始打扫，丙先帮着甲打扫，中途又去帮助乙打扫教室，最后两个教室同时打扫完，丙帮甲打扫了多长时间？(中途丙去乙教室用的时间忽略不计)
<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>