A project, according to the order of A, B, and C, will be completed in a whole number of days. If C, A, and B work in the order of one day each, the work will be completed 0.5 days later than the original plan. If B, C, and A are cycled in the order of one day each, the work will be completed one day later than the original plan. It will take 30 days for B to complete the project alone. Three people A, B, and C are doing it at the same time, how many days will it take to complete it?
一件工程，按甲、乙、丙各一天的顺序循环工作，恰需要整数天工作完毕．如果按丙、甲、乙各一天的顺序循环工作，比原计划晚0.5天工作完毕．如果按乙、丙、甲各一天的顺序循环工作，比原计划晚1天工作完毕．乙单独完成这件工程需要30天．甲乙丙三人同时做，需要多少天完成．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) * 2</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/30) + (1/15) + (1/30)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>