A beach is covered with gray, black, white, and spotted stones. Tara reaches down and grabs one stone without looking. Assume that the probability of her grabbing a gray stone is 5/14, a black stone is 3/14 and a white stone is 1/7.

What is the probability that she will grab a spotted stone?

Question

A beach is covered with gray, black, white, and spotted stones. Tara reaches down and grabs one stone without looking. Assume that the probability of her grabbing a gray stone is 5/14, a black stone is 3/14 and a white stone is 1/7. 

What is the probability that she will grab a spotted stone?

tkhunny · Answer

I don't really like the question, for starters.  She ALREADY grabbed a stone.  The probability is 1, if she grabbed a spotted stone, and 0 otherwise.

I don't really like the question, for seconders.  Are the stones scattered evenly or is she standing in a pile of grey stones?

Third, and finally, If there are only four colors, the probability of the fourth color must be whatever remains from the determination of the first three.  Remember, that for an entire distribution to work, all the probabilities must sum to unity.

Thus, 5/14 + 3/14 + 1/7 + p(Spotted) = 1 = 14/14