job, A and B alone need 20 days to complete, and C alone takes 15 days to complete. Now three people are working together. In the process of work, A asks for a day off due to something, and C takes a half-day rest due to illness. how many days?
一项工作，甲乙单独做各需要20天完成，丙单独做需15天完成，现三人合做，在工作过程中，甲有事请假一天，丙因病休息半天，问完成这项工作实际用多少天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(1/20) + (1/20) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / (1/6)</gadget>
<output>6</output>

<gadget id="calculator">6 + 0.5</gadget>
<output>6.5</output>

<result>6.5</result>