Let us assume that the number of pieces that can be cut = x
Length of each shelf board = 1 1/3 feet
= 4/3 feet
Total length of board available for making shelf = 18 foot
Then
4x/3 = 18
4x = 18 * 3
4x = 54
x = 54/4
= 13 2/4
As we can see from the above deduction, 13 shelf's can be made from the 18 foot board. There will be about 2/4th of the board remaining, but that 2/4th part will be wasted as it cannot create a complete board.I hope the answer is to your satisfaction and the explanation is clear to you.