Question

a carpenter has a board that is 8 feet long he cuts of 2 pieces one piece is 3 and a half feet long the other is 2 and one feet long how much of the board is left

Answer 1

3 1/2 I think
8-3 1/2=5 1/2
5 1/2-2=3 1/2

Answer 2

Well, assuming that the second piece is 2.5 feet long, we can set the expression up as 8-(3.5+2.5). 3.5+2.5=6, so the expression is 8-6. 8-6 is 2 so two feet of the board is left.
Hope this helps!
-Raiden