Given, A works twice as fast as B
Thus, the ratio of work done by A and B will be 2:1
Hence, ratio of time taken will be 1:2
Now, B can complete work in 12 days.
So, work done by B in one day = 1/12
Hence, work done by A is two times of B in one day = 1/6
work done by both A and B = 1/6 + 1/12 = 3/12 = 1/4
So, the number of days to finish the work = 4