A and B together can complete a piece of work in 15 days. B and C together can do it in 24 days. If A is twice as good a workman as C, then in how many days can B alone complete the work?

60 days

A + B = 15 days,

B + C = 24 days,

⇒ (A + B) - (B + C) = 8 - 5,

⇒ A - C = 3,     ..(1)

⇒ Also, given A : C = 2 : 1 

Put it in (1.)

⇒ (2 + 1)x = 3

⇒ x = 3,

⇒ Therefore, A = 2(3), C = 1(3) 

⇒ A : C = 6 : 3,  (Efficiency)

⇒ B + C = 5,

⇒ B + 3 = 5,

⇒ B = 2,

⇒ Time required by B to complete the total work alone = \(\frac{120}{2}\) = 60 days,    ..(\(\frac{Work}{Efficiency}\) = Time)