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?

$$\eqalign{ & {\text{Let the efficiency of C}} = {\text{x and A}} = {\text{2x}} \cr & {\text{A}} + {\text{B}} = {\text{8}} \cr & {\text{B}} + {\text{C}} = {\text{5}} \cr & \overline {\,{\text{A}} - {\text{C}} = 3\,} \cr & 2{\text{x}} - {\text{x}} = 3 \cr & {\text{x}} = 3 \cr & {\text{Efficiency of A}} = {\text{6, B}} = {\text{2, C}} = {\text{3}} \cr & {\text{Required answer}} \cr & {\text{B}} = \frac{{120}}{2} = 60{\text{ days}} \cr} $$