A project can be completed by A alone in 20 days and by B alone in 30 days. After 3 days of co-cultivation by teams A and B, team B will do the rest. How many days will it take to complete?
一项工程，甲单独做20天完成，乙单独做用30天完成．甲、乙两队合修3天后，余下的由乙队做，需要多少天才能完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</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">(1/12) * 3</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">(3/4) / (1/30)</gadget>
<output>45/2 = around 22.5</output>

<result>45/2 = around 22.5</result>