In a single row of yellow, green and red colored tiles,

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In a single row of yellow, green and red colored tiles, [#permalink]

01 Oct 2010, 09:12

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In a single row of yellow, green and red colored tiles, every red tile is preceded immediately by a yellow tile and every yellow tile is preceded immediately by a green tile. What color is the 24th tile in the row?
(1) The 18th tile in the row is not yellow.
(2) The 19th tile in the row is not green.

I don't get why it's not C.

[Reveal] Spoiler: OA

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Re: Green Yellow Red [#permalink]

01 Oct 2010, 09:34

here's the OA
It's from a MGMAT Test

"It is tempting to view the information in the question as establishing a pattern as follows:
Green, Yellow, Red, Green, Yellow, Red, . . .
However, consider that the following non-pattern is also possible:
Green, Yellow, Red, Green, Green, Green, Green . . .
(1) INSUFFICIENT: This tells us that the 18th tile is Green or Red but this tells us nothing about the 24th tile. Statement (1) alone is NOT sufficient.
(2) INSUFFICIENT: This tells us that the 19th tile is Yellow or Red but this tells us nothing about the 24th tile. Statement (2) alone is NOT sufficient.
(1) AND (2) INSUFFICIENT: Together, the statements yield the following possibilities for the 18th and 19th tiles:
GY, GR, RY, or RR
However, only GY adheres to the rules given in the question. Thus, we know that tile 18 is green and tile 19 is yellow. However, this does not help us to determine the color of the next tile, much less tile 24 (the one asked in the question). For example, the next tile (tile 20) could be green or red. Thus, the statements taken together are still not sufficient.
The correct answer is E."

But if we know that 19 is Yellow , then the nxt one will be red right ? What am I missing ?

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Re: Green Yellow Red [#permalink]

01 Oct 2010, 09:58

Barkatis wrote:

This is a tough question, I remember getting this in a MGMAT test and took me more than 3 minutes to think about it then. Let me break down my thought pattern :

What do we know ?
Y,G,R tiles
Whenever there is R, Y preceeds it and whenever there is Y, G preceeds it.
So if there is a R somewhere, it will always have a GYR pattern

What do we need to find out ?
Color of tile 24

Reasoning

(1) Tile 18 is not yellow.
This means tile 18 is either green or it is red.
Now it is easy to claim to different patterns which do not violate the rules :

18 ..... 24
RGGGGGG
RGGGGYR

So clearly not enough

(2) Tile 19 is not green
This means tile 19 can be red or yellow.
Consider the following patterns :

19 .... 24
RGGGGG
RGGGYR

So again not enough

(1+2) Now we know 18 is either G or R and 19 is either R or Y

18 ..... 24
GYRGGGG
GYRGGYR

Again very easy to form two patterns with different colors at 24

What was the basic principle I used ?
That the defining rules only specify restrictions looking right to left (preceeding tiles) for Y and R. So there is no restrictions on G either looking back or looking forward.

Answer is (E)
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Re: Green Yellow Red [#permalink]

01 Oct 2010, 12:00

Ah !!! I assumed that when a tile is yellow than the directly next tile will be red
but it just can be yellow also !
Thank you !

Re: Green Yellow Red [#permalink] 01 Oct 2010, 12:00

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In a single row of yellow, green and red colored tiles,

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In a single row of yellow, green and red colored tiles,

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