A and B together can do a piece of work in 6 days and A alone can do it in 13 days. In how many days can B alone can do it?  Choose the correct choice:
A) 12 days  B) 15 days  C) 78/7 days  D) 21 days  E) 22/7 days
Explanation:   A and B can do work 1/6 in 1 day   A alone can do 1/13 work in 1 day   B alone can do (1/6 -1/13) =7/78 work in 1 day   =&gt; complete work can be done in 78/7 days by B   Answer: Option 78/7 days
<result>C</result>
A and B together can do a piece of work in 6 days and A alone can do it in 13 days. In how many days can B alone can do it?