A project was completed in 15 days by the cooperation of A and B. Now A has completed it in 2 days and B in 5 days (7/30). If B alone does it, how many days will it take to complete it?
一项工程，甲乙合作15天完成，现在甲做了2天，乙做了5天，完成了(7/30)，如果由乙单独做，需要几天做完？
<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">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">(1/10) / 3</gadget>
<output>1/30 = around 0.033333</output>

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

<result>30</result>