A and B could do a piece of work in 12 days. B and C together do it in 15 days. If A is twice as good a workman as C, then B alone can do the work in

If A is twice as efficient as C, then if A's rate is $1/x$, C's rate is $1/(2x)$. Let B's rate be $1/y$. From the data, $\frac{1}{x}+\frac{1}{y}=\frac{1}{12}$ and $\frac{1}{2x}+\frac{1}{y}=\frac{1}{15}$. Subtracting gives $\frac{1}{2x}=\frac{1}{12}-\frac{1}{15}=\frac{1}{60}$, so $x=30$. Then $1/y=1/12-1/30=1/20$, hence B alone takes 20 days.