A is twice as efficient as B at completing a task, and together they can finish the task in 20 days. How many days would A alone take to complete the task?

Correct option is B

Let the efficiency of B = 1 unit per day 

So, Efficiency of A  = 2 unit per day 

All they together finish a work in 20 days, then total work  = (2+1)× 20 = 60 unit

Time taken by A to complete 60 unit of work = 60/2 = 30 days