B takes 12 more days than A to finish a task B and A start this task and A leaves the task 12 days b...........

Explanation:

Let us say A and B split their share of the task and started doing their respective shares simultaneously.
Let’s say A takes A days to finish the task. Therefore, B takes A + 12 days to finish the entire task.
A has to finish 40% of the task, since B is doing the rest. So A will only take 2A/5 number of days.
B only has to finish 60% of the task, so B will take (3(A+12)/5) number of days.
But as we know, B starts working along with A and finishes 12 days after A stops working.
So, (3(A+12)/5) = ((2A)/5+12)
3A + 36 = 2A + 60
A = 24; B = 36 days.