Hi All,
This question is more complex than a typical "sequence" question that you'll see on Test Day. The information at the beginning of the prompt is "causal" but only deals with tiles that PRECEDE other tiles; we have to deduce what tiles can FOLLOW other tiles. We know what comes immediately before a red (a yellow) and we know what comes immediately before a yellow (a green), but we're not told what comes before a green.
Up-Front Deductions:
A Green can be followed by another Green or a Yellow (but not a Red)
A Yellow can be followed by a Green or a Red (but not a Yellow)
A Red can be followed by a Green (but not a Yellow or Red)
We're asked for the 24th Tile....
Fact 1: 18th = NOT Yellow
This Fact tells us that the 18th = Green OR Red
IF....
18th = Red
We would have G, G, G, G, G to infinity, so the 24th Tile = GREEN
IF
18th = Green
We COULD have G, G, G, G, G, Y, so the 24th Tile = YELLOW
Fact 1 is INSUFFICIENT
Fact 2: 19th is NOT Green
This Fact tells us that the 19th = Yellow OR Red
IF
19th = Red
We would have G, G, G, G, G, so the 24th Tile = GREEN
IF 19th = Yellow
We could have R, G, G, G, Y so the 24th Tile = YELLOW
Fact 2 is INSUFFICIENT
Combined, we know...
18th = Green OR Red
19th = Yellow OR Red
Since Red can only be followed by GREEN, the 18th Term CAN'T be Red!
So, 18th = Green and 19th = Yellow
we could have G, G, G, G, G, so the 24th = GREEN
we could have G, G, G, G, Y, so the 24th = YELLOW
Combined, INSUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich