A and B can complete a work in 8 days. working together. B alone can do it in 12 days. After working for 4 days, B left the work. How many days will A take to complete the remaining work?

( A + B) 's 1 days work = 1/8
B's 1 days work = 1/12
∴ A's 1 day's work = (1/8) - (1/12) = (3 - 2)/24 = 1/24
∴ A can complete the work in 24 days
Now, B's 4 days work = 4/12 = 1/3
Remaining work = 1 - 1/3 = 2/3
As, time taken by A to complete the work is 24 days.
∴ Time taken by A to do 2/3 of the work = 2/3 x 24 = 16 days