An urn contains seven white balls and five black ones. Suppose $n$ balls are chosen at random. Let the random variable $X$ denote the number of white balls in the sample. What is the probability mass function of $X$ if the $n$ balls are chosen(a) without replacement?(b)with replacement?
I am basically not able to get a start for this question, like I am not sure how to define the sample space for this question. Again, it this question about using conditioned random variable function or using multinomial coefficent to get the possible ways of arranging the white balls?