Hi All,

This question can be solved by TESTing THE ANSWERS.

We're told that we have 9 blue cards and an unknown number of red cards. We're also told that if 2 cards are to be chosen at random from the stack WITHOUT replacement, then the probability that the cards chosen will BOTH be BLUE is 6/11. We're asked for the TOTAL number of cards.

Normally, when TESTing THE ANSWERS, we should start with either B or D. Answer B looks easier, so let's start there.

Answer B: 11 total cards

With 11 total cards, and 9 blue cards, the probability of pulling two blue cards is...

(9/11)(8/10) = 36/55

Since 6/11 = 30/55, this is clearly NOT the answer. The probability that occurs with Answer B is a little TOO BIG, so we need an answer that lowers the probability (and thus, requires MORE total cards...).

Answer C: 12 total cards

With 12 total cards, and 9 blue cards, the probability of pulling two blue cards is...

(9/12)(8/11) = 24/44 = 6/11

This is an exact MATCH for what we were told, so this MUST be the answer.

Final Answer:

GMAT assassins aren't born, they're made,

Rich

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# Rich Cohen

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