Number of days A required do a piece of work : 20

Number of days B required do a piece of work : 12

Work done by A in one day:

Work done by B in one day:

B works alone for 9 days, so work completed by B in 9 days : =

Work left =

But th of the work is already done by B

Time required to complete the remaining th of the work by A : 5 days

Time taken to finish the work 9 + 5 = 14 days

A can finish the remaining work in : 5 days