A project is completed in 12 days by cooperation between A and B. As a result, A worked for 3 days, and B worked for 1 day to complete the whole project (3/20). How many days can A work alone to complete this project.
一件工程甲乙合作12天完成，结果甲干了3天，乙干了1天完成了全工程的(3/20)，问这件工程甲一个人干几天能完成．
<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(3/20) - (1/12)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">(1/15) / 2</gadget>
<output>1/30 = around 0.033333</output>

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

<result>30</result>