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Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

We'll be glad if you participate in development of this project: 1. Please provide your solutions to the questions; 2. Please vote for the best solutions by pressing Kudos button; 3. Please vote for the questions themselves by pressing Kudos button; 4. Please share your views on difficulty level of the questions, so that we have most precise evaluation.

A jar contains 30 marbles, of which 20 are red and 10 are blue. If 9 of the marbles are removed, how many of the marbles left in the jar are red?

(1) Of the marbles removed, the ratio of the number of red ones to the number of blue ones is 2:1 --> since the total of 9 marbles are removed, then 6 red marbles and 3 blue marbles are removed, thus 20 - 6 = 14 red marbles are left in the jar. Sufficient.

(2) Of the first 6 marbles removed, 4 are red --> we don't know how many of the other 3 marbles removed were red. Not sufficient.

Re: A jar contains 30 marbles, of which 20 are red and 10 are bl [#permalink]

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06 Feb 2014, 12:05

A jar contains 30 marbles, of which 20 are red and 10 are blue. If 9 of the marbles are removed, how many of the marbles left in the jar are red?

(1) Of the marbles removed, the ratio of the number of red ones to the number of blue ones is 2:1. (2) Of the first 6 marbles removed, 4 are red.

Using st1, we can say that total no of marbles removed= sum of individuals marble removed which is equal to 2x:x for red : blue marble So we have 3x=9 or x=3 and thus remainig red marbles can be calculated A is sufficient St 2 says only for first 6 marbles in which 4 are red and the remaining 3 can be any ie all blue or all red or 1 red and 2 blue etc

So A alone is sufficient 600 level is okay

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Re: A jar contains 30 marbles, of which 20 are red and 10 are bl [#permalink]

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07 Feb 2014, 00:36

1

This post received KUDOS

No. of marbles = 30, of which Red, R = 20 & Blue, B = 10; Removed = 9; No. of marbles that will remain = 30 - 9 = 21;

No. of Red marbles remaining in the jar = ?

(1) Removed Marbles- R:B = 2:1; Therefore, 3x = 9; x = 3; So, since we know the no. of removed red marbles as 6, we can find the no. of remaining red marbles = 14; Sufficient;

(2) Insufficient, since we do not have information about the remaining 3 marbles that have been removed.

A jar contains 30 marbles, of which 20 are red and 10 are blue. If 9 of the marbles are removed, how many of the marbles left in the jar are red?

(1) Of the marbles removed, the ratio of the number of red ones to the number of blue ones is 2:1 --> since the total of 9 marbles are removed, then 6 red marbles and 3 blue marbles are removed, thus 20 - 6 = 14 red marbles are left in the jar. Sufficient.

(2) Of the first 6 marbles removed, 4 are red --> we don't know how many of the other 3 marbles removed were red. Not sufficient.

Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

We'll be glad if you participate in development of this project: 1. Please provide your solutions to the questions; 2. Please vote for the best solutions by pressing Kudos button; 3. Please vote for the questions themselves by pressing Kudos button; 4. Please share your views on difficulty level of the questions, so that we have most precise evaluation.

Thank you!

Although OG categorises it as Probability , I don't think this question will qualify as probability..

A jar contains 30 marbles, of which 20 are red and 10 are bl [#permalink]

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12 Aug 2015, 02:10

Bunuel wrote:

SOLUTION

A jar contains 30 marbles, of which 20 are red and 10 are blue. If 9 of the marbles are removed, how many of the marbles left in the jar are red?

(1) Of the marbles removed, the ratio of the number of red ones to the number of blue ones is 2:1 --> since the total of 9 marbles are removed, then 6 red marbles and 3 blue marbles are removed, thus 20 - 6 = 14 red marbles are left in the jar. Sufficient.

(2) Of the first 6 marbles removed, 4 are red --> we don't know how many of the other 3 marbles removed were red. Not sufficient.

A jar contains 30 marbles, of which 20 are red and 10 are blue. If 9 of the marbles are removed, how many of the marbles left in the jar are red?

(1) Of the marbles removed, the ratio of the number of red ones to the number of blue ones is 2:1 --> since the total of 9 marbles are removed, then 6 red marbles and 3 blue marbles are removed, thus 20 - 6 = 14 red marbles are left in the jar. Sufficient.

(2) Of the first 6 marbles removed, 4 are red --> we don't know how many of the other 3 marbles removed were red. Not sufficient.

Here not only we know the ratio of the marbles removed (red:blue = 2:1) but also that the number of removed marbles (9), so 6 red marbles and 3 blue marbles are removed.
_________________

A jar contains 30 marbles, of which 20 are red and 10 are bl [#permalink]

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28 Nov 2015, 18:06

Why is the second statement not sufficient? I set up a proportion like this: 4 Red Removed / 6 Removed = (x) Red Removed / 9 Removed and I got x = 6 which means 6 marbles were red out of the 9 marbles removed. I still dont get why is the second statement is not sufficent?

Why is the second statement not sufficient? I set up a proportion like this: 4 Red Removed / 6 Removed = (x) Red Removed / 9 Removed and I got x = 6 which means 6 marbles were red out of the 9 marbles removed. I still dont get why is the second statement is not sufficent?

Look at statement 2 this way.

You are given that total red = 20, total blue = 10. You have removed 9, out of which 4 are definitely red, 2 are blue. But you do not know anything about the remaining 3 balls.

If those 3 remaining balls are blue, you get 5 blue balls and 4 red balls removed, giving you the answer to the question asked = number of red balls remaining = 20-4 =16.

BUT, if those 3 remaining balls are red, you get 3 blue balls and 6 red balls removed, giving you the answer to the question asked = number of red balls remaining = 20-6 =14.

Additionally, you can create couple of other combinations for those 3 remaining balls giving you different answers for number of red balls. This makes statement 2 not sufficient.

In your analysis by creating the ratio of red balls to the total balls removed you are assuming that the ONLY case possible is for the proportion of red balls in the first 6 balls to remain the same for the remaining 3 balls. This is a massive assumption that is not supported either by the main question or by statement 2.

Your equation will not hold true if all 3 are blue of if the remaining 3 balls are 2 blue and 1 red or 1 blue and 2 red etc.

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