For a project, if A does it on the first day and B on the second day, and they do it alternately in turn, it will take exactly an integer number of days to complete it. If B does it on the first day and A does it on the second day, it will take more (2/5) days to complete it alternately. If A and B are allowed to cooperate at the same time, it only takes 2 (5/8) days to complete. Now, how many days will it take for B to do it alone?
一项工程，如果第一天甲做，第二天乙做，这样交替轮流做，恰好用整数天完成．如果第一天乙做，第二天甲做，这样交替轮流做要多(2/5)天才能完成．如果让甲、乙两人同时合作，只需2(5/8)天就可以完成．现在，由乙独做需要多少天才能完成？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / (1/7)</gadget>
<output>7</output>

<result>7</result>