Ans.
Time taken by A to complete a work alone - 24 days
Time taken by A in completing 1/3 of work = 24*1/3= 8 Days
8 day = time taken by B in completing 1/2 of work
B alone can complete the work = 16 days
The required time in completing the work if both A & B start working together?
16*24 / 40 = 48/5 Days