A project can be completed by A alone in 20 hours, and B alone in 30 hours. Now A works first for 12 hours and B continues. How many hours will it take to complete?
一项工程，甲单独做20小时完成，乙独做30小时完成，现由甲先做12小时后，乙接着做，还需多少小时完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">12 * (1/20)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">(2/5) / (1/30)</gadget>
<output>12</output>

<result>12</result>