For a manuscript, the time required for A, B, and C to play alone is 20 hours, 24 hours, and 30 hours respectively. Now the three are fighting together, but A withdraws early because of another task on the way, and it takes 12 hours to complete, how many hours did A spend?
一份稿件，甲、乙、丙三人独打需要的时间分别是20小时、24小时、30小时．现在三人合打，但甲因途中另有任务提前撤出，结果用12小时完成，甲打了多少小时？
<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

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

<gadget id="calculator">(1/24) + (1/30)</gadget>
<output>3/40 = around 0.075</output>

<gadget id="calculator">(3/40) * 12</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) / (1/20)</gadget>
<output>2</output>

<result>2</result>