A and B can do a piece of work in 12 days while A alone can do it in 20 days. How long will B alone take to finish the work?

In 12 days, A and B can do 1 job.

In 1 day, A and B can do 1/12 job.

Again,

In 20 days, A can do 1 job

In 1 day, A can do 1/20 job

Thus, In 1 day, B alone can do (1/12 - 1/20) job

                                       = (10-6)/120 job

                                       = 4/120 job

                                       = 1/30 job

Now,