A and B together can do a works in 12 days. B and C together do it in 15 days. If A's efficiency is twice that of C, then the days required for B alone to finish the work is

A and B will do in 1 day, $$\frac{1}{12}$$ amount of work
B and C will do in 1 day, $$\frac{1}{15}$$ amount of work
Hence adding up above two equations
A+2B+C will do in 1 day, $$\frac{9}{60}$$ amount of work
as it is mentioned A=2C (i.e. efficiency of A is twice of C)
So 2C+2B+C will do in 1 day $\frac{9}{60}$$ amount of work
or 2(B+C) + C = $\frac{9}{60}$$
or C = $$\frac{1}{60}$$ (as B+C = $$\frac{1}{15}$$ )
or B = $$\frac{1}{15}$$ - $$\frac{1}{60}$$ = $$\frac{1}{20}$$ 
So B alone will do work in 20 days.