Let's suppose you have a row $n$ with the first square yellow, then $m$ green squares, followed by $20-m$ yellow squares. What would the pattern be on the next row? We know the first two squares are one yellow and one green. Since in the row $n$ you have green squares till column $m+1$, the square on row $n+1$ will be yellow, from column $3$ till column $m+1$. The next square on row $n$ will be green, since you have the transition from green to yellow in the above column, followed by all yellow squares, till the end of the row. So you have yellow, green, many yellows, one green, many yellows. That's two copies of the first row. These will evolve independently, until there is a green square from the first structure replacing touching the first green in the second structure. In the solution image, it means that on row 9 you have the first 8 elements in row 1, followed by a copy of row 1. So each structure will evolve the same as the one from row 1 down, until the square in column 9 is green. This will happen on row 16. Note that at that point you have 8 green squares from the structure on the left, followed by 8 green squares from the structure on the right, so 16 green squares. Then in next row, you will have two copies of row 1, one starting at column 1, one at column 17.
