If A and B work together to complete a project, it takes 10 days to complete it, and B alone takes 15 days to complete it. How many days will it take A to complete it alone?
一项工程，甲、乙合做需10天完成，乙单独做要15天完成，甲单独做要多少天完成．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

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

<result>30</result>