A and B together can complete a piece of work in 4 days. If A alone can complete the same work in 12 days. in how many days can b alone complete that work?

A.  1/3 days   
B.  1/6 days
C.   1/9 days   
D.  1/12 days
Answer:     B

Explanation :

(A+B)'s 1 day's work    = 1/4
A's 1 day's work    = 1/12
B's 1 day's work    = (1/4 - 1/12)
                        = 1/6.
Hence, B alone can complete the work in 6 days.