Explanation:

Solution: 
A completes 40% of work in 12 days so 60% of the work has to be completed by A and B.
They together take 12 days to complete 60% of work.
So working together, they can complete whole work in 12/60 × 100= 20 days
A completed 40% work in 12 days, he can complete whole work in 12/40 × 100 = 30 days
Let B takes x days to complete whole work. So,
1/30 + 1/x=1/20
Solving we get x = 60
Now A completes whole work in 30 days. B in 60 days so A is twice or 100% efficient than B.