The two masters and apprentices work together to make a batch of parts, which can be completed in 12 days. The master did it for 3 days first, and then went out for business, and the apprentice continued to do it for 1 day. The task was completed (3/20). If the master did it alone, how much God can complete the task?
师徒二人合做一批零件，12天可以完成，师傅先做了3天，因事外出，由徒弟接着做1天，共完成任务的(3/20)，如果让师傅单独做，多少天能完成任务？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<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">2 / (1/15)</gadget>
<output>30</output>

<result>30</result>