Question

A can do a piece of work in 20 days which B alone can do in 12 days. B worked at it for 9 days. A can finish the remaining work in how many days?

Answer 1

B does the job in 12 days
so he does 1/12 of the job in 1 day.
so he does 9/12 of the job in 9 days = 3/4 of the job.
balance job will be 1/4 
...
A does the job in 20 days
so he does 1/20 of the job in 1 day.
1/4 of the job he will take 
..
(1/4)/(1/20)
1/4 *20 =5 days

Answer 2

A can do the remaining work in 5 days