Here is a general formula for solving this problem. Call the two sides
of the rectangle a and b. Then
a + b - (a² + b² - ab)^½
---------------------------
6
equals the side of the little square pieces to cut away. The
measurements given were 8 ft. by 3 ft., and the above rule gives 8 in.
as the side of the square pieces that have to be cut away. Of course it
will not always come out exact, as in this case (on account of that
square root), but you can get as near as you like with decimals.