posted by anuj .
a bag contains 7 red balls, 4 green balls and 5 yellow balls. what is the probability that 3 balls drawn at random are either green or yellow?
If the balls are either green or yellow, it means that we do not want any red balls to be drawn out of the 16 balls.
Assuming the balls are drawn at random AND WITHOUT REPLACEMENT, for the first ball, we have 9 choices out of 16. The second: 8 out of 15, and the third: 7 out of 14.
So P(3 G∪Y)=(9/16)(8/15)(7/14)
If the drawn balls are replaced, then the probability that none of the balls is red will be
P(3 G∪Y with replacement)=(9/16)^3