A and B can finish a work in 20 days. A alone can do 1/5th of the work in 12 days. In how many days can B alone do it?

The correct answer is: 30 days.

Let’s say that the total work can be represented as W and let’s say the amount of work that A can do in one day is W_A and the amount of work that B can do in one day is W_B.
Step 1 of 3:
It is given that A alone can do 1/5th of the work in 12 days.
So, Work done by A in one day(W_A) =  = …….(1)
Step 2 of 3:
It is given that A and B can finish a work in 20 days. So we can write
W_A   20 + W_B  20 = W
Using equation (1)
 + W_B  = 
W_B  = 
Step 3 of 3:
Now, the time taken by B complete  amount of work = 1 day
So, time taken by B to complete W amount of work =  = 30 days
Final Answer:
Hence, the time that will B to complete the whole work is 30 days.