project can be completed by A alone in 20 days and by B in 30 days. Now A and B will cooperate for a few days first. The remaining B is transferred away, and A works for another 5 days to complete the project. Find the number of days for A and B to cooperate.
一项工程，甲独做20天完成，乙独做30天完成．现在先由甲、乙合作若干天后．剩下的乙调走，甲又做了5天，完成了工程，求甲、乙合作的天数．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 5</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

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

<gadget id="calculator">(3/4) / (1/12)</gadget>
<output>9</output>

<result>9</result>