Answer:
28
STEP BY STEP:
Imagine that each person is assigned one wall, and all 777 people begin painting at the same time.
Since everyone will finish painting their assigned wall after 282828 minutes, it takes one person 282828 minutes to paint one wall.
If we have 141414 people and 141414 walls, we can again assign one wall to each person.
Everyone will take 282828 minutes to paint their assigned wall.
In other words, it takes 282828 minutes for 141414 people to paint 141414 walls.