A. 4 days
Feedback That is, if b alone can finish the work in 12 days,
A alone can finish the work in 6 days.
If A can complete a work in x days and B can complete the same work in y days, then, both of them together can complete the work in x y/ x+ y days.
A and B can together finish the work in 6 × 12/18 days = 4 days.