A project, if A and B work together, it will take 15 days to complete, B alone will take 20 days to complete, how many days will it take A to complete alone?
一项工程，甲，乙合做需要15天完成，乙单独做需要20天完成，甲单独做要几天完成？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<result>60</result>