This is a tricky question...

every R must be preceeded by a Y
every Y must be preceeded by a G

however, there is no restriction on what must preceed G or what must succeed R. if this is understood, then we can clearly see why the answer is E.

I persume when you combine both statements, you will get:

..
..
18th - G
19th - Y
..
..

even if the 20th tile is Red and the 21st is Green, there is no guarantee that the 22nd must be Yellow. It could be another Green. In fact, tiles 21 to 24 could all be greens.

Or, tiles 21 to 23 could be green and tile 24 be yellow.
Or, tiles 21 to 22 be green and tile 23 be yellow and tile 24 be red.

Insufficient. E.

-BM-