In the sixth puzzle, the water needs to be lowered six feet, which requires five columns. Since there are eight places to put the columns, you won't need three of them. Thus there are fifty-six possible combinations of steps. Here is a picture of all of the possible positions of columns and their heights.

Here is a picture of all of the possible positions of columns and their heights. The horizontal line segments represent the top of the column, while the number above each segment is how tall that column would be. Remember, to solve the puzzle, you need one column on each level, and the columns should step down, from left to right, one foot at a time.

First, we do not have any 1-foot or 2-feet columns, thus these possibilities can be eliminated.

If we try using both 3-foot columns to make a 6-foot column for the bottom row, we'll need more 5-foot columns than we have to complete the rest of the ladder.

Thus, we have to use a 4-foot column in the bottom row. This eliminates more possibilities.

Let's try using both 3-foot columns to make a 6-foot column again, this time for the second row from the top (on the left). In order to do this, we’d have to use the 5-foot column on the top row. This means we'd be left with only 4-foot columns, which is a problem. Notice that on the middle row, we must use either a 3-foot or 5-foot column.

So we can't use both 3-foot columns to make a 6-foot column. This also eliminates using a 3-foot column in the middle row. Thus, we must use the 5-foot column in the middle row.

Since we just used our only 5-foot column, there aren't any left to use along the top row. Thus, we must put a 3-foot and a 4-foot column together to make a 7-foot column. The remaining columns fall into place after this.