It takes 15 days for a batch of mechanical parts to be completed by the cooperation of A and B. Now after A has made 4 days, B will continue to make 2 days, and a total of (7/30) of the total amount of this batch of parts will be completed. If this batch The parts are made by A alone, how many days will it take to complete?
一批机械零件，由甲、乙两人合作要15天完成，现在由甲做了4天后，再由乙接着做2天，共完成这批零件总量的(7/30)，如果这批零件由甲单独做，需要多少天才能完成？
<gadget id="calculator">7 / 30</gadget>
<output>7/30 = around 0.233333</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) * 2</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">(7/30) - (2/15)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">(1/10) / 2</gadget>
<output>1/20 = around 0.05</output>

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

<result>20</result>