A project can be completed in 30 hours by A alone, and 20 hours by B alone. If A works 10 hours first, and the remaining two work together, how many hours will it take to complete?
一项工程，甲独做需30小时完成，乙独做需20小时完成，甲先做10小时后，余下的二人合做，还需多少小时完成？
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) * 10</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(2/3) / (1/12)</gadget>
<output>8</output>

<result>8</result>