Suppose B alone takes x days to finish the work. Then, A alone can finish it in (x - 6) days.
Now, (A’s one day’s work) + (B’s one day’s work)
and  Â
∴                 Â
        Â
        Â
  But, x cannot be less than 6.     Â
So, x = 12.
Hence, B alone can finish the work in 12 days.