A and B can do a piece of work in 18 days. B and C together can do it in 30 days. If A is twice as good a workman as C, find in how many days B alone can do the work?

A is twice as efficient as C, so if C's rate is c, A's rate is 2c. Let B's rate be b. A + B = 1/18, B + C = 1/30, and A = 2C. Substituting: 2C + B = 1/18 and B + C = 1/30. Subtracting: C = 1/18 - 1/30 = (5-3)/90 = 2/90 = 1/45. So C alone takes 45 days, A takes 45/2 = 22.5 days. B = 1/30 - 1/45 = (3-2)/90 = 1/90, so B alone takes 90 days.