rxs0005 wrote:

a deck of cards has 2 cards of blue , green , orange , red total 8 cards

what is the Probability of drawing two cards that are NOT blue

The answer is

6/8 * 5/7 = 15/28

My Q is why cant we use the negation logic to get the same answer

that is 1 - ( both blue cards )

1 - 2/8 * 1/7 = 27 /28 why is this wrong ?

Both answers are correct, but they're answers to different questions. 15/28 is the probability that *neither* card is blue. 27/28, on the other hand, is the probability that the cards are not *both* blue. The second answer is much larger than the first, because it includes all of the ways in which you might pick one blue card and one non-blue card.

_________________

Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.

Private GMAT Tutor based in Toronto