A mechanical processing operation can be completed in 5 days with 4 A-type machine tools; 3 days with 4 A-type machine tools and 2 B-type machine tools; 2 days with 3 B-type machine tools and 9 C-type machine tools It can be completed. If one of the three types of machine tools is used for 5 days, and the remaining A and C machine tools continue to work, how many days will it take to complete the work.
一项机械加工作业，用4台A型机床，5天可以完成；用4台A型机床和2台B型机床3天可以完成；用3台B型机床和9台C型机床，2天可以完成，若3种机床各取一台工作5天后，剩下A、C型机床继续工作，还需要多少天可以完成作业．
<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 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/15) + (1/30)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 5</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/4) / (1/12)</gadget>
<output>3</output>

<result>3</result>