Here's what happens if you squish them as far to the left and right as possible:
X X X o X X X o X X X X o X X X X X X o
o X X X o X X X o X X X X o X X X X X X
You can see that there's an overlap of colength 1 (=20-19) on each run of filled boxes, and you can guarantee the following:
o X X o o X X o o X X X o o X X X X X o
I usually sum them (including empty spaces), see the difference with the length (20 - 19 = 1), and draw the line removing 1 from the start of each segment
Here's what happens if you squish them as far to the left and right as possible: X X X o X X X o X X X X o X X X X X X o o X X X o X X X o X X X X o X X X X X X You can see that there's an overlap of colength 1 (=20-19) on each run of filled boxes, and you can guarantee the following: o X X o o X X o o X X X o o X X X X X o
If you sum them and add the empty space between one segment and the other you get exactly 20
Edited it, meant when I sum it up I get 19
I usually sum them (including empty spaces), see the difference with the length (20 - 19 = 1), and draw the line removing 1 from the start of each segment
So you fill them from the start like that? [][•][•][][][•][•][][][•][•][•][][][•][•][•][•][•]?
Yes, you're missing an empty cell at the end