In a stack of cards, 9 cards are blue and the rest are red. If 2 cards are to be chosen at random from the stack without replacement, the probability that the cards chosen will both be blue is 9/11. What is the number of cards in the stack?  Choices:
A) 10	B) 11	C) 12	D) 15	E) 18
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 9/11. 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 A: 10 total cards   With 10 total cards, and 9 blue cards, the probability of pulling two blue cards is...   (9/10)(8/9) = 72/90~ 9/11   10
<result>A</result>
In a stack of cards, 9 cards are blue and the rest are red. If 2 cards are to be chosen at random from the stack without replacement, the probability that the cards chosen will both be blue is 9/11. What is the number of cards in the stack?