The price of lumber is calculated by the board foot. One board foot is
the amount of lumber in a board 12" long, 12" wide, and 1" thick, or the
equivalent. Below we show other dimensions equal to one board foot.

1" X 12" X 12"
2" X 6" X 12"
2" X 4" X 18"

The lumber yard sells one type of lumber with the end dimensions of 1" by 6".
What length of 1" by 6" lumber is equal to one board foot?