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 from the following answers
A) 12 days	B) 15 days	C) 78 / 7 days	D) 21 days	E) 22 / 7 days

Rationale: "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 = > complete work can be done in 78 / 7 days by b answer : option c"
Answer: C