A job needs 10 hours to be completed by A alone, 15 hours to be completed by B alone, and 20 hours to be completed by C alone. Now after 2 hours of cooperation between A and B, A leaves due to something, and after 3 hours, C joins in until the work Finish. How many hours did it take to complete this job.
一件工作，甲单独完成需10小时，乙单独完成需15小时，丙单独完成需20小时，现在甲乙合作2小时后，甲因有事离开了，又过3小时后，丙加入进来，直到工作完成．完成这件工作共用了多少小时．
<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">2 * (1/6)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">3 * (1/15)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/3) - (1/5)</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/15) / (7/60)</gadget>
<output>4</output>

<gadget id="calculator">2 + 3 + 4</gadget>
<output>9</output>

<result>9</result>