Pipes A and B together can fill a tank in 10 hours. Pipes B and C together can fill the same tank in 12 hours. Pipes C and A together can fill the same tank in 15 hours. In how many hours can pipe B alone fill the same tank?

120/7 hours 

Total Capacity = 60l

Adding the given efficiencies:

(A + B) + (B + C) + (C + A) = 6 + 5 + 4

⇒            2(A + B + C)         =     15

⇒              (A + B + C)         =     $\frac{15}{2}$ --------(i)

Efficiency of B = eff(A + B + C) - eff( A + C)

                     = $\frac{15}{2}$ - 4    (from eq.(i))

                     = $\frac{7}{2}$        ----------(ii)

Time taken by pipe alone to fill the tank of capacity 60l = $\frac{Capacity}{efficiency}$

                                               = $\frac{60 \times 2}{7}$       

                                               = $\frac{120}{7}$ hours