A is twice as efficient as B.

A and B together can complete the task in 20 days. 

Total work = Efficiency $$\times$$ Time

Let the efficiency of A be 2x and B's efficiency be x unit per day

Total work = (2x+ x)$$\times 20$$ = 60x

Time taken by A to complete the work = $$\frac{60x}{2x} =30\ \ days$$