A piece of work can be completed in 12 days by master-student cooperation, 30 days by apprentice alone, and how many days by master alone.
一件工作，师徒合做12天可完成，徒弟独做30天可完成，师傅独做多少天可完成．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

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

<gadget id="calculator">(1/12) - (1/30)</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>