Two borders opposite each other are 12 feet in length and 1 foot wide. There are 2 walls in between that are 10 feet long and 1 foot wide. The total number of feet used is 10 + 10 + 12 + 12 = 44 feet of brickwork.
The number of bags needed = 44/9 = 4 8/9 = 4.9 bags. <<<<< Answer rounded to the nearest tenth.
In practical terms, you would best buy 5 bags. You sure won't have much left.