The board A is 8 inches. If you use 3/4 of an 8-foot-board. Then you're left with 2 feet.
3 6
-- = --
4 8
8 - 6 =2
(8= the length of the board)
1/3 of 2 feet = 8 inches
1 24 24
-- x -- = --- = 8 inches
3 1 3
(24= the equal amount of inches in a 2-foot-board)