A and B can do a job together in 12 days. A is 2 time as efficient as B. In how many days can B alone complete the work?

Time take by A to complete a work = N days
∴ Time taken by B to complete the same work = 2N days
According to the question,
(1/N) + (1/2N) = 1/12
⇒ 3 / 2N = 1/12
∴ N = (3 x 12)/2 = 18
Time taken by B to complete the same work = 2N= 2 x 18 = 36 days