Explanation

Person A completes the work in 12 days, so their work rate is 1/12 of the work per day.

Person B completes the work in 6 days, so their work rate is 1/6 of the work per day.

When they work together, their combined work rate is the sum of their individual work rates:

(1/12 + 1/6) = (1/12 + 2/12) = 3/12 = 1/4 of the work per day.

Since they can complete 1/4 of the work in a day, they will complete the entire work in: 4 days.