A and B can do a piece of work in 20 days. B and C together can do it in 40 days. If A is thrice as good a workman as C, find in how many days B alone can do the work?

80 days

A     :     C = 3     :     1

Make total work equal

(A + B) × 20 = (B + C) × 40

         A + B = 2B + 2C

        A - 2C = B

        3 - 2 × 1 = B ⇒ B = 1

Total work = ( 3 + 1 ) × 20 = 80

Time taken by B = \(\frac{80}{1}\) = 80 days