A project was completed in 12 days by master-student cooperation, and the actual master rested for 5 days. This project took 15 days to complete. How many days would it take for the master to complete this project alone?
一项工程，师徒合做12天完成，实际师傅休息5天，这项工程用了15天完成，师傅单独完成这项工程需要多少天？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<gadget id="calculator">(1/12) * 10</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) / 5</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>